[icoscp-core] Make exceptions easier to handle

[squaregoldfish]

All errors thrown from the library appear to be basic Exceptions, so if we want to know what happened programmatically we have to parse the text of the exception message, e.g.:

Exception: SPARQL SELECT problem, HTTP response code: 503, response: You have exceeded your per-minute query quota. Please wait 1 minute to run more queries.

It would be much easier to have a QuotaExceededException or something similar in this instance.