Add `-fmax-errors` flag

This commit adds a command line option to limit the number of errors
displayed. It also moves the documentation for `reverse-errors` into the
`Warnings` section.

https://ghc.haskell.org/trac/ghc/ticket/13326

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3323