dev/buildchecker: attempt to run all reporters before exiting

Created by: bobheadxi

With our growing number of reporters, it would be less than ideal if one failed and all the others failed as well. Additionally, the number of reporting implementations is making main.go hard to read, so we extract them to more isolated functions.

This refactor is mostly geared towards https://github.com/sourcegraph/sourcegraph/pull/38020

Test plan

yolo merge and fix if broken