Improve rustls pemfile parsing errors

[itsjunetime]

This improves the error messages that come from parsing pemfiles with rustls.

I ran into the previous error when trying to parse a file that actually returned an empty iterator when fed into rustls_pemfile::read_all - I got an error that said 'private key format not supported', but the actual issue was that I had no keys in the provided file.

This PR changes the error messages so that this situation would return an error stating The private key file contained no keys. It also returns more specific errors when it contains a known format that we can't process (by explicitly stating the type in the error message) and also informs the user if their file somehow contains multiple keys (where we expect it to contain just one).

This also avoids the allocation of the intermediate Vec which is supposed to contain just the one key from the file.