fail Client build if Identity + default backend + !cfg(native-tls)

[eric-seppanen]

If ClientBuilder::build encounters a TlsBackend::Default, and cfg(native-tls) is not enabled, it doesn't know how to load the client certificate. Rather than silently ignore the attempt to use a client Identity, return an error.

This fix feels a little unsatisfying, for a couple of reasons:

1. The problem might come back. It's not obvious which cfg gates should be applied at the various places Identity gets touched, so the code seems fragile in this area.
   Perhaps there should be a way to indicate that the Identity was successfully consumed? Then if we reach the end of build() and we still have an unused identity, we know there's a problem and should return an error.
2. There's no test coverage. I'm looking into how to do this, but I'm not there yet. So far all I can offer is a flimsy "works for me."

I think this fixes #903, though it contains a few different reports over the years and it's hard to tell if they're all the same thing.

[cpu]

@seanmonstar How do you feel about this branch's proposed solution to #903 ?

I think this fixes #903, though it contains a few different reports over the years and it's hard to tell if they're all the same thing.

We ran into this problem debugging a user bug report over in the Rustls repo (see my comment on 903 for more details). I was able to confirm the patch in this branch produces a helpful error for the situation we debugged.

Can I do anything to help move this fix (or an alternative) along?

[seanmonstar]

I'm always grateful for better errors, so thank you! I see your comment in the issue, and agree it'd be best if the different formats could be made to work with whatever back-end is selected. I'll include this for now, though.

[cpu]

Thanks!