I think we should add some message to the tool's verbose text if an HTTPS proxy is in use and CURLE_SSL_CACERT. For example append a line mentioning --proxy-insecure and --proxy-cacert if the user specified an HTTPS proxy:
curl performs SSL certificate verification by default, using a "bundle"
of Certificate Authority (CA) public keys (CA certs). If the default
bundle file isn't adequate, you can specify an alternate file
using the --cacert option.
If this HTTPS server uses a certificate signed by a CA represented in
the bundle, the certificate verification probably failed due to a
problem with the certificate (it might be expired, or the name might
not match the domain name in the URL).
If you'd like to turn off curl's verification of the certificate, use
the -k (or --insecure) option.
HTTPS proxy has similar options --proxy-cacert and --proxy-insecure.
@jay, you should not ask me but IMHO, ideally, curl should detect/relay which certificate (origin or proxy) failed verification and adjust the error message accordingly. Reaching that ideal is probably difficult, and your change is probably sufficient to help most users.
Right, we don't return enough accuracy in the return code for a user to tell if it was a server or proxy that failed the certificate verification - that might be a subject for a separate issue. So until we have that, I'm 👍 on this suggestion.