[flang][driver] Add checks for errors from `Prescan` and `Parse`

If either `Prescan` or `Parse` generate any fatal errors, the new driver
will:
  * report it (i.e. issue an error diagnostic)
  * exit early
  * return non-zero exit code
This behaviour is consistent with f18 (i.e. the old driver).

Reviewed By: sameeranjoshi

Differential Revision: https://reviews.llvm.org/D93712