[errors] Introduce error registration API

[ejgallego]

Partially addresses #7560

[This PR is a WIP, in particular the implementation of the registration is still not complete, thus I'm looking for comments in the API]

Coq exception handling is complex, and requires quite a bit of
meta-data on exceptions, including custom printing and documentation
for each error.

We introduce an error registration API that should help with the above
problematic, by requiring new errors to be properly registered.

There are some questions:

• OCaml has support for private extensible types, however that cannot
  be used with exceptions unless we introduce a CErrors.t type. May
  be worth doing.

• We could make the error kinds extensible too, but I'm not sure it is
  worth the gain, and it could be used badly.

• Is the E0 variant worth it?

A new error is declared as:

module UE_Sig = struct
  type t = string option * Pp.t
  let print (hdr, pp) = Pp.strpp
  let doc = Pp.str "Generic User Error, to be deprecated"
  let kind = ErrorKind.Regular
end

module UserError = CoqError.Make(UE_Sig)

in the .mli file:

module UserError = CErrors.S with type t := string option * Pp.t

the with type constraint is needed if the user wishes to expose the
exception constructor.

See https://github.com/ejgallego/coq/tree/errors%2Bconstructor_port for an example of porting.

• Corresponding documentation was added / updated (including any warning and error messages added / removed / modified).
• Entry added in the changelog (see https://github.com/coq/coq/tree/master/doc/changelog#unreleased-changelog for details).

[coqbot]

For your complete information, the following job in allow failure mode has failed: test-suite:4.12+trunk+dune

[ejgallego]

By the way, I remember the the reason I wanted to refine exceptions as those that carry some context, IDEs and serialization may want to handle those differently [long story, but that's the reason]

[coqbot-app]

The "needs: rebase" label was set more than 30 days ago. If the PR is not rebased in 30 days, it will be automatically closed.

[coqbot-app]

The "needs: rebase" label was set more than 30 days ago. If the PR is not rebased in 30 days, it will be automatically closed.

[SkySkimmer]

With no progress for years let's close