Add flag -raise-embedded-errors to Driver

[jaymody]

Adds a flag -raise-embedded-errors to the shared args in Driver, which will raise the first ocaml.error node found in the processed AST.

Motivation

It's useful to be able to run a standalone ppx driver for it's side affects (e.g. registering correction files), without actually making use of it's output directly.

For example, given the program program.ml:

type t = int [@@deriving_inline my_deriver]

One might run:

$ my_ppx.exe program.ml -null

With the objective of registering a correction file while ignoring any output. However, since errors are embedded into the AST, the error [%%ocaml.error "ppxlib: [@@@deriving.end] attribute missing"] never gets surfaced to the users. Now it can be, with -raise-embedded-errors.