Check for message destination endpoint validity

[trevorhardy]

Currently, any string can be used as the destination in the helicsEndpointSendBytesTo() and being inattentive to the parameter order could lead to accidentally specifying an invalid destination endpoint. There is no check in pyhelics for the validity of the endpoint specified in the API call and thus users are able to send out a message to an invalid destination.

Though there are error messages generated in the broker log, they are generic ("no route for message") and more specific error messages stating that the defined endpoint is not valid would be helpful.

I'll leave it up the the developers to determine if this should be implemented in the language bindings or if an update to the core library is needed.