Accessing non-existing property is a common action (for example
hasattr() do try to access the property). So, introduce specific
exception, inheriting from AttributeError. It will behave very similar
to standard (non-Admin-API) property access.
This exception is reported to the Admin API user, so it will be possible
to distinguish between non-existing property and access denied. But it
isn't any significant information leak, as list of valid properties is
publicly available in the source code.
QubesOS/qubes-issues#853
