Return different error code for SolverProblemsException

To make it easier for external tools to detect SolverProblems and react
to them accordingly,
this PR introduces a new exit code.
Merge pull request #1827 from pulse00/exitcode-patch

Return different error code for SolverProblemsException
Workaround when your proxy server fails request with enabled http_proxy_request_fulluri HTTP parameter
updated diagnostics to test fulluri proxy param handling
Merge branch 'request_fulluri'
Fix CS and wording, remove lowercased env var, add env var to docs