Raise better exception on FASP error responses (#35262)

app/lib/fasp/request.rb

@@ -49,6 +49,8 @@ class Fasp::Request
   end
 
   def validate!(response)
+    raise Mastodon::UnexpectedResponseError, response if response.code >= 400
+
     content_digest_header = response.headers['content-digest']
     raise Mastodon::SignatureVerificationError, 'content-digest missing' if content_digest_header.blank?
     raise Mastodon::SignatureVerificationError, 'content-digest does not match' if content_digest_header != content_digest(response.body)