Show Problem message returned by Central Backend

[matthew-white]

Expected Behaviour

If Central Backend returns an error response in its standard "Problem" format, I'd expect Web Forms to show the message contained within the Problem object.

Observed Behaviour

In getodk/central#2219, Backend returns a 404 error. The response data is a standard Problem object. However, Web Forms indicates that there was no response:

Something went wrong: there was no response to your request.

Reproduction Steps

1. Navigate to https://test.getodk.cloud/projects/749/forms/styletest/submissions/new
2. Click Send

Screenshots or Videos

Environment

See getodk/central#2219.

Additional Context

getodk/central#2219 describes a bug in Central Backend. However, it also shows that Web Forms isn't surfacing the error from Backend.