Clean up prebuilts update script

Improves failure and commit messages, bring code style in line with
PEP. Does not affect system image or SDK build.

Test: ./update_current.py -p -s 4270497
Bug: 64661766
Change-Id: I276a06eaabdd61d4a1d4c0a9a712b2b3fc1b2002
(cherry picked from commit cd3de266d4463919fc2bb5d9325821ebbdf0d6c9)
1 file changed