Better error reporting for the `SERVICE` operation

[UNEXENU]

This commit implements better understandable error messages for SERVICE clauses, in particular for the following cases:

• The remote endpoint responds with a status code other than 200 OK
• The remote endpoint responds with a content type other than application/sparql-results+json, which is the only content type thant we currently support. To implement these improvements, the HttpClient class has been extended s.t. the sendRequest method not only returns the body of the response, but also the status code and the content typ header.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.36066% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.45%. Comparing base (be816a9) to head (48c6116). Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/util/http/HttpClient.cpp	97.56%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1452 +/- ## ========================================== + Coverage 89.41% 89.45% +0.03% ========================================== Files 346 346 Lines 25350 25385 +35 Branches 3381 3383 +2 ========================================== + Hits 22667 22707 +40 + Misses 1494 1490 -4 + Partials 1189 1188 -1 ```

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[sonarcloud[bot]]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud