Description of problem:

Validation errors on API requests have a different response format from other errors (e.g. resource not found). We should consistently use one format for reporting API errors to client applications.
  
Actual results:

Validation errors are reported as

<errors>
  <error>
    <message>Catalog name has already been taken</message>
  </error>
</errors>


Expected results:

Validation errors should be reported as

<error>
  <code>ValidationError</code>
  <message>Catalog name has already been taken</message>
</error>