Fixed issue where we attempted to remove CommandSet from a list it was not in

Author: kmvanbrunt

cmd2/cmd2.py

@@ -458,7 +458,8 @@ def install_command_set(self, cmdset: CommandSet) -> None:
         except Exception:
             for attrib in installed_attributes:
                 delattr(self, attrib)
-            self._installed_command_sets.remove(cmdset)
+            if cmdset in self._installed_command_sets:
+                self._installed_command_sets.remove(cmdset)
             raise
 
     def _install_command_function(self, command: str, command_wrapper: Callable, context=''):