Handle missing prefixes more elegantly

[albertmeronyo]

Currently we are just catching (thanks @c-martinez ) errors when the query uses undeclared prefixes. This is very common.

At the moment the backend fails silently, and simply no OpenAPI spec is rendered for the offending query prefixes. Using the UI to at least show some error (and suggestions for fixing) would be a great addition.

Another option would be to intelligently try to guess the prefix, by e.g. keeping a hard-coded list of common prefixes, issuing a request to prefix.cc, etc.

[albertmeronyo]

There is now a warning about missing prefixes in #337

[c-martinez]

I don´t like the idea of intelligently guessing prefixes, because it changes the query behaviour. For example if you take the same query and run it directly against the SPARQL endpoint, you would get a different result. I don't think grlc should make such changes to queries.

Can we close this issue?