Fix error reporting in the test server.
Fix error reporting in the test server.

We were still using the error protobuf field in the response message,
but this has meanwhile been replace by HTTP status codes.

BUG=none
TEST=toy server successfully generates error response upon invalid/unspecified DM tokens in policy requests

Review URL: http://codereview.chromium.org/6883224

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@83494 0039d316-1c4b-4281-b951-d872f2087c98

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