Adds a catch for HandshakeEception
Adds a catch for HandshakeEception which is thrown when there is an error validating the server's SSL certificate. Before this would be caught under the catch all exception which indicates that the server is completely offline. Resolves #3 (closed)
Note to maintainer: I am not particularly well versed in flutter/dart development, but this seemed to be a fairly straightforward change, and was tested to work on my Pixel3A.