Errors in browsable API on error.

[tomchristie]

Closes #3024.
Refs #2926.

Example:

[jpadilla]

@tomchristie just wondering, why the changes needed in _force_text_recursive()?

[tomchristie]

@jpadilla - We're raising the errors as ValidationErrors. The force_text_recursive allows the internationalized strings to be realized into a particular language, but we're losing the associated backlink to the serializer in the process.

[tomchristie]

Can't lie - the form error handling isn't the most graceful, but we do what needs to be done. One area for gradual improvement once the big functionality pieces are done & dusted.