Minor get/setPath exception handling improvement.

Locatable.(get|set)AttributeValue were both overreaching a little bit in
terms of what exceptions to swallow.

Improve Locatable.getAttributeValue to only swallow one exception (for
unknown properties, which is considered legit by unit tests and returns
null).

Improve Locatable.setAttributeValue to not swallow any exceptions. Before
this change, setting a non-existing path was silently ignored!