Added httpcore.SSLError and redirect TLS handshake errors to it.

[T-256]

Summary

Discussed at encode/httpx#3214.
httpcore.SSLError is subclass of httpcore.ConnectError, so it's backward compatible.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[stale]

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