I think Milo interpreted the failure from Swarming (Note that swarming says "failure" instead of "infra failure" because technically it was not an infra issue with swarming).

Milo didn't receive any signals from Annotee about what the exit status should be, therefore the swarming status took hold.  If the recipe engine emitted a status, that status would've taken precedence.

This bug is that this step should've been marked as an exception, and the recipe engine should've emitted an exception annotation:
@@@SEED_STEP@Uncaught Exception@@@
@@@STEP_CURSOR@Uncaught Exception@@@
@@@CURRENT_TIMESTAMP@1487718152.71@@@
@@@STEP_STARTED@@@
@@@STEP_LOG_LINE@exception@Traceback (most recent call last):@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/run.py", line 508, in _old_run@@@
@@@STEP_LOG_LINE@exception@    recipe_result = recipe_script.run(api, properties)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 81, in run@@@
@@@STEP_LOG_LINE@exception@    self.run_steps, properties, self.PROPERTIES, api=api)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 554, in invoke_with_properties@@@
@@@STEP_LOG_LINE@exception@    **additional_args)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/loader.py", line 515, in _invoke_with_properties@@@
@@@STEP_LOG_LINE@exception@    return callable_obj(*props, **additional_args)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/recipes/chromium_trybot.py", line 41, in RunSteps@@@
@@@STEP_LOG_LINE@exception@    return api.chromium_tests.trybot_steps()@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 578, in _inner@@@
@@@STEP_LOG_LINE@exception@    return func(*a, **kw)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 889, in trybot_steps@@@
@@@STEP_LOG_LINE@exception@    disable_deapply_patch) = self._trybot_steps_internal()@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 578, in _inner@@@
@@@STEP_LOG_LINE@exception@    return func(*a, **kw)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 915, in _trybot_steps_internal@@@
@@@STEP_LOG_LINE@exception@    bot_update_step, bot_db = self.prepare_checkout(bot_config_object)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 578, in _inner@@@
@@@STEP_LOG_LINE@exception@    return func(*a, **kw)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 210, in prepare_checkout@@@
@@@STEP_LOG_LINE@exception@    bot_config.initialize_bot_db(self, bot_db, update_step)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/recipe_modules/chromium_tests/bot_config_and_test_db.py", line 76, in initialize_bot_db@@@
@@@STEP_LOG_LINE@exception@    chromium_tests_api.get_compile_targets_for_scripts()@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 578, in _inner@@@
@@@STEP_LOG_LINE@exception@    return func(*a, **kw)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 854, in get_compile_targets_for_scripts@@@
@@@STEP_LOG_LINE@exception@    ] + self.get_common_args_for_scripts(),@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 578, in _inner@@@
@@@STEP_LOG_LINE@exception@    return func(*a, **kw)@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/recipe_modules/chromium_tests/api.py", line 813, in get_common_args_for_scripts@@@
@@@STEP_LOG_LINE@exception@    properties[name] = self.m.properties[name]@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_modules/properties/api.py", line 36, in __getitem__@@@
@@@STEP_LOG_LINE@exception@    return self._properties[key]@@@
@@@STEP_LOG_LINE@exception@  File "/b/swarming/w/irqR8mDk/build.git/checkout/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/types.py", line 66, in __getitem__@@@
@@@STEP_LOG_LINE@exception@    return self._d[key]@@@
@@@STEP_LOG_LINE@exception@KeyError: 'slavename'@@@
@@@STEP_LOG_END@exception@@@
@@@CURRENT_TIMESTAMP@1487718152.71@@@
@@@STEP_CLOSED@@@

Milo is WAI here, this is a recipe issue.

nodir, I think you're working on this? Speculatively assigning to reflect reality, please mark available if you're not working on it.

yes. Once swarmbucket receivse kitchen result, it will be able to distinguish failures from infra failures