Use enumerations for Error kinds

[aesteve]

For a newcomer implementing Error Messages, choosing the right kind of error is a bit difficult. What is "kind" ? What does it mean ? Is it documented somewhere ? Which one should I pick ?

A very simple first step could be to gather all the existing kinds into an enumeration, this is simple, and allows to gather every kind of error in a single place.

This would allow newcomers to, at least, quickly get a glimpse of the different kind of errors already used (Syntax, Reference, etc.).

After that, maybe documenting the enum would be a very good idea.

[propensive]

Looks good. Could we call them "categories" of errors, rather than "kinds"?

[som-snytt]

-Wconf nomenclature is "category".