Fix bad error messages on seed comment submission

[metasoarous]

Expected behavior:
If there's an error with seed comment submission, such as a failed call to the Google Translate API, a simple message should be displayed like "Unable to translate comment. Please contact administrator."

Actual behavior:
An html response is sent, which is then rendered as an undecipherable string.

Screenshots:

Additional context:

• Worth considering whether we should still accept the comment in this case, though we need to know lang for later translation steps, so maybe best to continue blocking here. Then again, could always backfill once translation is working again.
• This may also affect comment submission from the participation UI.