gluon-web: improve error handling of parse_message_body()

Actually raise an error and turn it into an HTTP 400 return code when
something goes wrong, rather than ignoring the error.

We also improve the conditions under which errors are thrown before
pump() is called: We don't need to check for the multipart/form-data
content-type twice, and a POST without this content-type is now always
an error.

(cherry picked from commit f3960eeb)