Add better error messages to the webkit_tests step in the android buildbot code.

R=ilevy@google.com, ilevy@chromium.org, peter@chromium.org
BUG=

Review URL: https://codereview.chromium.org/27197012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229193 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229193