Example:
https://chromium-swarm.appspot.com/task?id=3ba75afa6c924b10

5012 2018-02-13 16:36:58.631 E: Error while removing named cache u'C:\\b\\s\\w\\ir\\cache\\git' at u'git'. The cache will be lost.
Traceback (most recent call last):
  File "C:\b\s\swarming_bot.1.zip\client\run_isolated.py", line 1241, in install_named_caches
    named_cache_manager.uninstall(path, name)
  File "C:\b\s\swarming_bot.1.zip\client\named_cache.py", line 212, in uninstall
    name, path, ex))
Error: cannot uninstall cache named u'git' at u'C:\\b\\s\\w\\ir\\cache\\git': [Error 5] Access is denied
5012 2018-02-13 16:36:58.678 E: Error while removing named cache u'C:\\b\\s\\w\\ir\\cache\\vpython' at u'vpython'. The cache will be lost.
Traceback (most recent call last):
  File "C:\b\s\swarming_bot.1.zip\client\run_isolated.py", line 1241, in install_named_caches
    named_cache_manager.uninstall(path, name)
  File "C:\b\s\swarming_bot.1.zip\client\named_cache.py", line 212, in uninstall
    name, path, ex))
Error: cannot uninstall cache named u'vpython' at u'C:\\b\\s\\w\\ir\\cache\\vpython': [Error 5] Access is denied
5012 2018-02-13 16:36:58.726 E: Error while removing named cache u'C:\\b\\s\\w\\ir\\cache\\builder' at u'webrtc_win'. The cache will be lost.
Traceback (most recent call last):
  File "C:\b\s\swarming_bot.1.zip\client\run_isolated.py", line 1241, in install_named_caches
    named_cache_manager.uninstall(path, name)
  File "C:\b\s\swarming_bot.1.zip\client\named_cache.py", line 212, in uninstall
    name, path, ex))
Error: cannot uninstall cache named u'webrtc_win' at u'C:\\b\\s\\w\\ir\\cache\\builder': [Error 5] Access is denied


I'm filing this bug mostly because it fills up the failure rate on Swarming. bot_update takes 6 hours, which means it's deeply broken.

Assigning to Edward for triage.