Change CheckCallError to inherit from Error. This will simplify try/except changes.

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/3324007

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@58695 0039d316-1c4b-4281-b951-d872f2087c98