Fix CloudMonitorCheck.get

Fix ``CloudMonitorCheck.get`` so that it properly calls ``CloudMonitorCheckManager.get`` instead of the non-existent ``CloudMonitorCheckManager.get_check``.

Catch AttributeErrors raise during calls to get() from ``BaseResource.__getattr__`` and re-raise them as non-AttributeError to avoid implying that the Resource doesn't have the attribute just because some other attribute reference in the get() wasn't found.

``CloudMonitorEntity.list_checks`` now calls ``set_entity(self)`` on each returned check. 
I don't believe it's correct to effectively downcast an `AttributeError` only to reference `AttributeError` in the message text. We can re-raise `AttributeError` with more detail if needed, but we shouldn't go more generic to mask it.