From https://chromium-swarm.appspot.com/task?id=3bfff5a075b3ec10&refresh=10&show_raw=1&wide_logs=true:

4592 2018-03-02 15:08:57.904 E: Error while removing named cache u'e:\\b\\s\\w\\ir\\cache\\builder' at u'builder_46f3a1e165a23d5ebe36064e42ad1dcc384350146515d216f8a5bed564fe7109'. The cache will be lost.
Traceback (most recent call last):
  File "e:\b\s\swarming_bot.1.zip\client\run_isolated.py", line 1241, in install_named_caches
    named_cache_manager.uninstall(path, name)
  File "e:\b\s\swarming_bot.1.zip\client\named_cache.py", line 212, in uninstall
    name, path, ex))
Error: cannot uninstall cache named u'builder_46f3a1e165a23d5ebe36064e42ad1dcc384350146515d216f8a5bed564fe7109' at u'e:\\b\\s\\w\\ir\\cache\\builder': [Error 5] Access is denied

The underlying bot has actually died due to a hanging compile step. This error is non-critical, but can easily become red herring when debugging issues.