Improve error handling for remote resources

[mmarx]

Error handling for remote resources (e.g., SPARQL queries, but this also applies to HTTP resources) should be improved. Right now, most HTTP error codes will just panic – we should turn this into a nice error message.

[mmarx]

We should also handle SPARQL timeouts (and other errors) more gracefully – usually these will have some non-TSV data among the TSV stream.

[mmarx]

Another failure mode that we should have a diagnostic for is CORS failures on remote requests (this will probably only happen in nemo-web).