Remove sys.exit calls

[kenodegard]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

These sys.exit calls in conda-build are very painful for downstream users of the code base as a library (i.e., conda-forge). The reason is that in the python exception hierarchy has the SystemExit exception raised by sys.exit and the standard Exception at the same level (https://docs.python.org/3/library/exceptions.html#exception-hierarchy). This means when one calls sys.exit, the invoking code cannot catch that exception without resorting to explicitly catching SystemExit as opposed to a normal Exception. The slight change in exception handling API breaks (at least my) expectations as a user of the code and has created quite a few mystery code crashes in the bot over the years.

Can we use a standard exception here and then have the conda-build CLI code handle that exception and call sys.exit?

Originally posted by @beckermr in #5237 (comment)

Why is this needed?

sys.exit calls are harder to catch/debug when using conda_build as an API.

What should happen?

Review and ideally remove all sys.exit calls.

Additional Context

No response

[kenodegard]

There are 88 sys.exit calls in conda-build

[kenodegard]

Oops, this is a duplicate of #4209