Changing exit to raise for not overridden test, to match the change made in 83043003

BUG=316397

Review URL: https://codereview.chromium.org/83703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236826 0039d316-1c4b-4281-b951-d872f2087c98

tools/cr/cr/commands/command.py

@@ -100,6 +100,5 @@ class Command(cr.Plugin, cr.Plugin.Type):
       context: The context to run the command in.
     """
     _ = context
-    print 'Command "{0}" not implemented'.format(self.name)
-    exit(1)
+    raise NotImplementedError('Must be overridden.')