Differentiate between different kinds of errors

[stefandesu]

Continuation of #36.

There should be different error messages depending on the kind of error. In particular, we should differentiate between 404 (not found), 5xx (backend reachable, but other error), and network errors.

[nichtich]

Should also be used when using curl to directly get data:

curl -I https://coli-conc.gbv.de/terminology/XXX/?format=json

returns 504 but I'd expect 404.