Fix raise to use Exception instead of raw strings
Fix raise to use Exception instead of raw strings

I coded these incorrectly in a previous change and hit them when
building a VS package.

Review URL: https://codereview.chromium.org/1567863004

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@298156 0039d316-1c4b-4281-b951-d872f2087c98

primiano/depot_tools,CoherentLabs/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,primiano/depot_tools