Change sys.exit to Raise.

[erykoff]

The fix for #1688 in #1761 breaks help("modules") introspection and also leads to unhappy results when inadvertently importing blackd from python. E.g.:

$ python
Python 3.8.10 | packaged by conda-forge | (default, May 11 2021, 07:01:05) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> help('modules')

Please wait a moment while I gather a list of all available modules...

aiohttp dependency is not installed: No module named 'aiohttp'. Please re-install black with the '[d]' extra install to obtain aiohttp_cors: `pip install black[d]`
$

(Notice that python has been exited).

This PR changes the sys.exit(-1) in the import code to a proper Raise.

[ichard26]

Ah yeah this makes sense. Thanks for the PR!

[zsol]

Lgtm, just one minor comment

[ichard26]

Not waiting for all of primer since the chance of them uncovering an issue here is extremely low.

[ichard26]

Hey thanks for your contribution! This project is only possible by contributions like these 🖤. You're awesome, @erykoff, never really thought about how a sys.exit would be problematic on import (not sure why, but eh who cares :p). If you have any suggestions about your contributing experience here, please let us via #2238, thank you once again!