Stop trying to munge return codes from subprocess

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

The existing logic doesn't really provide much additional value and does break google-ability.

For example, prophet has several reports about an error code 3221225657, which has basically 0 hits on google besides prophet's repo. That's because we're assuming the world is a unix system and subtracting 128. The real error is 3221225785, which has significantly more helpful results.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):
Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[syclik]

LGTM!