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Adds a new pyauto exception for failures caught by automation calls (inherits from JSONInterfaceError).
Improves the error messages generated for automation infrastructure failures.

TEST=Manually caused automation commands to fail and verified error messages.
BUG=132803

Review URL: https://chromiumcodereview.appspot.com/10855161

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@151853 0039d316-1c4b-4281-b951-d872f2087c98