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>

command.py

@@ -16,11 +16,11 @@ import multiprocessing
 import os
 import optparse
 import re
-import sys
 
 from event_log import EventLog
 from error import NoSuchProjectError
 from error import InvalidProjectGroupsError
+from error import RepoExitError
 import progress
 
 
@@ -42,6 +42,10 @@ WORKER_BATCH_SIZE = 32
 DEFAULT_LOCAL_JOBS = min(os.cpu_count(), 8)
 
 
+class UsageError(RepoExitError):
+    """Exception thrown with invalid command usage."""
+
+
 class Command(object):
     """Base class for any command line action in repo."""
 
@@ -215,7 +219,7 @@ class Command(object):
     def Usage(self):
         """Display usage and terminate."""
         self.OptionParser.print_usage()
-        sys.exit(1)
+        raise UsageError()
 
     def CommonValidateOptions(self, opt, args):
         """Validate common options."""