Update errors to extend BaseRepoError

In order to better analyze and track repo errors, repo command failures
need to be tied to specific errors in repo source code.

Additionally a new GitCommandError was added to differentiate between
general git related errors to failed git commands. Git commands that opt
into verification will raise a GitCommandError if the command failed.

The first step in this process is a general error refactoring

Bug: b/293344017
Change-Id: I46944b1825ce892757c8dd3f7e2fab7e460760c0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/380994
Commit-Queue: Jason Chang <jasonnc@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Tested-by: Jason Chang <jasonnc@google.com>
Reviewed-by: Joanna Wang <jojwang@google.com>

subcmds/__init__.py

@@ -16,6 +16,7 @@ import os
 
 # A mapping of the subcommand name to the class that implements it.
 all_commands = {}
+all_modules = []
 
 my_dir = os.path.dirname(__file__)
 for py in os.listdir(my_dir):
@@ -42,6 +43,7 @@ for py in os.listdir(my_dir):
         name = name.replace("_", "-")
         cmd.NAME = name
         all_commands[name] = cmd
+        all_modules.append(mod)
 
 # Add 'branch' as an alias for 'branches'.
 all_commands["branch"] = all_commands["branches"]