Handle ssl.SSLWantWriteError

[mnightingale]

Even when reading from a socket it’s possible to get an SSL_ERROR_WANT_WRITE error.

Reference: https://docs.openssl.org/master/man3/SSL_read/

If the underlying BIO is nonblocking, a read function will also return when the underlying BIO could not satisfy the needs of the function to continue the operation. In this case a call to SSL_get_error(3) with the return value of the read function will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE. As at any time it's possible that non-application data needs to be sent, a read function can also cause write operations.

Both are non-fatal, but we need to make the Python code aware of them.

I also made the exception messages match what Python would usually raise.