Change sys.exit to raise ImportError (#2440)

The fix for #1688 in #1761 breaks help("modules") introspection and also leads
to unhappy results when inadvertently importing blackd from Python. Basically
the sys.exit(-1) causes the whole Python REPL to exit -- not great to suffice.

Commit history before merge:

* Change sys.exit to Raise.
* Add #2440 to changelog.
* Fix lint error from prettier
* Remove exception chain for more helpful user message.

Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>