Rework setting HTTP errors in CUPS (fixes #893)

Rework setting HTTP errors in CUPS (fixes #893)

Sometimes errno is not set when we want to report HTTP error, so we
should use `http->error` if available or internal server error. In cases
of internal HTTP related errors where we don't have HTTP connection
available (before setting of HTTP connection or in callbacks which
process IPP messages), use `_cupsSetError()`.

Fixes #893