Set MySqlException error number when SSL exception happens

[bgrainger]

When SslStream.AuthenticateAsClientAsync fails, a MySqlException with error code 0 is thrown. It should be MySqlErrorCode.UnableToConnectToHost.

Note that this will break some clients that are currently detecting error code 0: #621 (comment)

In general, all exceptions along the connection path should use UnableToConnectToHost.

[bgrainger]

Adding MySqlErrorCode.UnableToConnectToHost in many more places, except when the client connection string is clearly bad (e.g., specifying a client key file that doesn't exist); these seem like "more fatal" errors that can't be fixed by retrying. There didn't seem to be an existing appropriate MySqlErrorCode value for these exceptions.

[bgrainger]

Fixed in 0.57.0.