Use sys.exit(...) instead of exit(...): exit(...) should not be used in programs

Summary:
Backport 3-of-6 for Core PR10781
T473
Depends on D2150

Test Plan: N/A as these scripts are not used. This is a dependency for other backports.

Reviewers: deadalnix, schancel, #bitcoin_abc, Fabien

Reviewed By: schancel, #bitcoin_abc, Fabien

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2151