HypernetX specific warnings should be in hypernetx classes

[drj11]

Is your feature request related to a problem? Please describe.

As per Tutorial 1, calling edge_distance (H.edge_distance(4, 6, s=2)) can generate a warning.

A mild improvement would be to make the warning a specific class in hypernetx (as the Exceptions are); this gives finer control over warnings to users.

(i do not require this to be fixed for my JOSS review)