Add better error message when trying to get edge that is not present

[finsberg]

When trying to get an edge that is not present in the graph it will give a KeyError, but I think it would be better to have an error message saying that the edge is not present in the graph.

[rossbar]

I think a better exception message would be an improvement, though changing the exception type would be a backwards-incompatible change. One possible compromise would be to re-raise the KeyError with the new exception message (i.e. replace the nx.NetworkXError in this PR with KeyError).

This would give the better exception message while preserving back compat - WDYT?

[finsberg]

I think a better exception message would be an improvement, though changing the exception type would be a backwards-incompatible change. One possible compromise would be to re-raise the KeyError with the new exception message (i.e. replace the nx.NetworkXError in this PR with KeyError).

This would give the better exception message while preserving back compat - WDYT?

Or, if it is important that all exceptions that are emitted from NetworkX is of type nx.NetworkXError, we could create a new exception that is a subclass of KeyError and nx.NetworkXError, i.e

class NetworkXKeyError(KeyError, nx.NetworkXError)
    pass

[rossbar]

if it is important that all exceptions that are emitted from NetworkX is of type

IMO this isn't important, but perhaps others have more informed opinions. I'd prefer to stick with builtin exceptions where possible, and I think KeyError works nicely for this case (NX graphs are "dicts all the way down" after all). I can be persuaded otherwise though; the only thing I'm -1 on is a backward-incompatible change in the exception type!

[finsberg]

Sounds good. I changed it to re-raise KeyError. Also casting the exception to a string didn't work (see python/cpython#116408 (comment)) so that is why I changed it to use e.value.args instead.

[rossbar]

I took the liberty of pushing a few changes (mostly cosmetic) and tried to simplify the match= logic a bit by making more liberal use of wildcards.

Otherwise this LGTM, thanks @finsberg !

[dschult]

Looks good... Thanks!