Use Mastodon's JSON error-documents.

Use Mastodon's JSON error-documents.

When a Mastodon API request returns an HTTP error status, it generally
also delivers a JSON error-document containing a string. This is a
much better error message than the plain HTTP code, because often it
goes into more useful detail. For example, if you have an auth failure
because you haven't confirmed the email address on your new account
yet, the HTTP error code can't tell you anything about email address
confirmation, but the error-document will. So we'll use those messages
in preference, if they're available.