When Curl tries to fetch the rewrite test URL and something goes wrong the user
has no clue what was the problem. This problems can include:
- The rewriting not working at all.
- HTTPS redirects do not work.
- Curl does not accept the presented SSL cert.
This commit fixes this by providing the Curl error message to the user to further
debug the problem.
Fixes #3629.~
Hypolite Petovan
