bad_revision needs to be checked before passing to RunSteps, sometimes resulting in exception later:

@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\recipe_engine\recipe_engine\run.py", line 397, in run@@@
@@@STEP_LOG_LINE@exception@    recipe_result = recipe_script.run(api, api._engine.properties)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\recipe_engine\recipe_engine\loader.py", line 57, in run@@@
@@@STEP_LOG_LINE@exception@    self.RunSteps, properties, self.PROPERTIES, api=api)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\recipe_engine\recipe_engine\loader.py", line 512, in invoke_with_properties@@@
@@@STEP_LOG_LINE@exception@    **additional_args)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\recipe_engine\recipe_engine\loader.py", line 473, in _invoke_with_properties@@@
@@@STEP_LOG_LINE@exception@    return callable_obj(*props, **additional_args)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts/slave\recipes\findit/chromium/compile.py", line 119, in RunSteps@@@
@@@STEP_LOG_LINE@exception@    root_solution_revision=bad_revision)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\recipe_engine\recipe_engine\recipe_api.py", line 220, in _inner@@@
@@@STEP_LOG_LINE@exception@    return func(*a, **kw)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts/slave\recipe_modules\chromium_tests\api.py", line 203, in prepare_checkout@@@
@@@STEP_LOG_LINE@exception@    bot_config, root_solution_revision, force=force)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\recipe_engine\recipe_engine\recipe_api.py", line 220, in _inner@@@
@@@STEP_LOG_LINE@exception@    return func(*a, **kw)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts/slave\recipe_modules\chromium_tests\api.py", line 166, in ensure_checkout@@@
@@@STEP_LOG_LINE@exception@    force=force)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\recipe_engine\recipe_engine\recipe_api.py", line 220, in _inner@@@
@@@STEP_LOG_LINE@exception@    return func(*a, **kw)@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\depot_tools\recipe_modules\bot_update\api.py", line 89, in ensure_checkout@@@
@@@STEP_LOG_LINE@exception@    root = cfg.solutions[0].name@@@
@@@STEP_LOG_LINE@exception@  File "E:\b\build\scripts\slave\.recipe_deps\recipe_engine\recipe_engine\config.py", line 483, in __getitem__@@@
@@@STEP_LOG_LINE@exception@    return self.data.__getitem__(index)@@@
@@@STEP_LOG_LINE@exception@IndexError: list index out of range@@@

Example builds:
https://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_variable_webkit_layout/builds/20
https://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_variable_webkit_layout/builds/21