Consider RestClient throw on real network errors

[detunized]

Currently RestClient doesn't throw at all. This is by design. This way it's easy to handle non-2XX HTTP status codes. On the flip side, we have to manually handle network errors everywhere. Normally those are thrown right away and never handled by the library. Usually these errors indicate problems like lack of connectivity or DNS failure. No reason to handle this in the lib. Let the app handle it and just throw directly from RestClient. This would also make a lot of tests unneeded.