Add periods after API error strings.

Some of the error strings had periods, and some didn't. I haven't yet checked
djblets' errors, but this makes the reviewboard errors consistent.
Add DIFF_PARSE_ERROR to the ValidateDiffResource.create error list.

This corrects the errors listed in the docs.
Don't expose child resources in ValidateDiffResource.

For convenience, ValidateDiffResource inherited from DiffResource.
This brought along the child resources, which weren't valid to have.
That ended up breaking docs, which tried to traverse them.