Translate the error code displayed in debug logs

The mailing list contains many examples of people being confused by the mysterious message:

Returned error: XX

I myself find it pretty annoying to have to look into Constants.pm whenever I'm debugging somebody's issue.

This commit makes the portal error constant appear in debug logs, for easier grepping around:

Returned error: 5 (PE_BADCREDENTIALS)

@guimard I could use your advice on the changes I made in Constants.pm . I don't fully understand how the whole "exporting" mechanism works and whether or not I may have broken something here. Unit tests say I didn't, but I could use a second opinion after modifying something so fundamental.