Refine error messaging for generate-assertions

[vteague]

• Consider conveying the error messages, rather than just the code, in the case of an error. Perhaps the 'reason' field of the http header is the right way to do this.
• Consider how to deal with time_out_trimming_assertions, which isn't actually an error, but possibly deserves to be communicated back to colorado-rla. Possibly we just add a specific header, or we have raire_warning, or something.

[vteague]

Superseded by other changes