lib/fetcher-curl: Prefix fatal errors with full URL
lib/fetcher-curl: Prefix fatal errors with full URL

Just include the whole URL that failed if libcurl failed with something
elementary like CURLE_COULDNT_CONNECT or CURLE_COULDNT_RESOLVE_HOST.

Closes: #1731

Closes: #1732
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree