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Provides the ability to generate error messages within schema-compiled code for ease of debugging. 
Error messages may be disabled by adding a 'generate_error_messages' property to json schema, e.g. "generate_error_messages": false

Error generation mostly written by Aaron Jacobs (https://codereview.chromium.org/16462004/).

BUG=234834

Review URL: https://chromiumcodereview.appspot.com/22228002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@217118 0039d316-1c4b-4281-b951-d872f2087c98