Fix exception type in DeviceProxy.__getattr__ if attribute was removed

Closes #677 (closed)

An exception should still be raised when trying to access a Tango attribute that previously existed, but has since been removed. However, we must raise AttributeError, not DevFailed.

Unfortunately, there is no good way to invalidate attribute cache, except for wrapping call into try-except. In case of the successful call, it slows down PyTango for couple ns for each operation, but I think we can live with this.