cerbero: Don't wrap custom.py loading in a try..except

1. It always exists
2. We want it to always be loaded
3. It swallows all exceptions and that should be avoided at all costs
7 jobs for dont-use-tarballs-gstci in 211 minutes and 57 seconds (queued for 38 minutes and 18 seconds)
Status Job ID Name Coverage
  Preparation
passed #164950
manifest

00:00:47

 
  Build
passed #164951
cerbero deps fedora x86_64

01:01:50

passed #164952
cerbero deps windows x86

01:00:23

passed #164953
cerbero deps windows x86_64

00:55:40

 
  Full Builds
passed #164955
build cerbero cross win32

00:13:03

passed #164956
build cerbero cross win64

00:09:24

passed #164954
build cerbero fedora x86_64

00:12:34