Harden stratis-cli to prevent stack traces from missing D-Bus properties

[mulkieran]

With zbus, it is more possible that the GetManagedObjects result will not have certain properties for an object path present. When an expected property is not present, a stack trace results.

This can effect the examination of a GetManagedObjects result.

We do this two ways:

1. Via the MO{Pool,Dev,Filesystem} classes which access these properties.
2. When looking for a specific pool object path based on its Name or Uuid property.

For the first, it might help for dbus-client-gen methods to allow specifying a default value in case the property is non-existent.

For the second, catching and hiding the KeyError exception would be the correct way to go.