Register custom handlers for exceptions

[brendanlong]

Some OCaml exceptions have special information which we would like to represent in Sentry in ways besides as an sexp string. For example, Async has monitor.Error, which contains a much more useful backtrace in Async code, exceptions thrown by Error.t sometimes also contain backtraces, and other exceptions could probably be parsed into an exception type + title + description instead of merging all of that into a sexp.

An example interface (a not particularly efficient one) is something like:

val register_exn_converter : exn -> Sentry.Exception.t option

let () =
  register_exn_converter (function
     | My_custom_exception (title, description) -> Some ...
     | _ -> None)

Then we could iterate over all of these until we get a Some result, and fall back to sexp version if necessary. We probably also want to expose the default version so implementers can use that, then just override the parts they can do better.

If we do this, we can remove the custom logic for montitor.Error and Error.t and just register converters for them by default.