Raise repo exit errors in place of sys.exit

Bug: b/293344017
Change-Id: I92d81c78eba8ff31b5252415f4c9a515a6c76411
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/381774
Tested-by: Jason Chang <jasonnc@google.com>
Reviewed-by: Joanna Wang <jojwang@google.com>
Commit-Queue: Jason Chang <jasonnc@google.com>

tests/test_error.py

@@ -21,12 +21,16 @@ import unittest
 import error
 import project
 import git_command
+import fetch
+import command
 from subcmds import all_modules
 
 imports = all_modules + [
     error,
     project,
     git_command,
+    fetch,
+    command,
 ]