Do not forcibly exit on ExitCode.Success

[rossabaker]

Motivations:

• Spark cluster exits with code 16 when sys.exit is called. This makes "successful" jobs fail.
• Returning main on success, rather than forcibly exiting, allows the natural shutdown for the platform to proceed. For example, this gives any non-daemon threads a chance to finish their jobs on the JVM.

[rossabaker]

/cc @kaiserpelagic, who has been battling this with fs2.StreamApp.

[alexandru]

JS is always tricky. I guess at this point the keep-alive was canceled.

But yes, this is better for JS as well.

[codecov-io]

Codecov Report

Merging #252 into master will decrease coverage by 0.12%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
- Coverage   89.57%   89.44%   -0.13%     
==========================================
  Files          57       57              
  Lines        1544     1544              
  Branches      153      149       -4     
==========================================
- Hits         1383     1381       -2     
- Misses        161      163       +2