Show error details in tags of endpoint's JSON on parse failure

[sknick]

I discovered that it's incredibly difficult to figure out what is wrong with your docstring's YAML when it fails to parse. This pull request adds details about the parse failure to the tags of the relevant endpoint. Since the additional lines are in the tags, they don't cause a further validation failure.

[sknick]

Note that the checks failing weren't because of my code change.