Make "job create" return an error if the job wasn't created successfully.

"job create" now calls the api check_status method, and generates an error message like the following if it doesn't find the created job:

     Error occurred while creating job west/bozo/test/hello

Bugs closed: aurora-886

Reviewed at https://reviews.apache.org/r/27082/
Add a script to update build-support/pants_requirements.txt.
Ran that script and it generated a slightly different output (no more
system dependencies).

Also fix a bug in the pants script that causes us to bootstrap pants
on every run (missing $PANTS_VERSION).

Testing Done:
Ran the update requirements script and ran pants multiple times to
confirm it doesn't bootstrap.

Reviewed at https://reviews.apache.org/r/27097/
Simplify the ResourceSlot class.
Checkstyle fixes.
Appease computer.