Propagate errors in the on_slash hook of loans

[daniel-savu]

Is your feature request related to a problem? Please describe.
Due to the slash signature in FRAME, the orml-traits OnSlash hook is also not allowed to return a Result.

Describe the solution you'd like
The opened issue on the substrate repo seems to be slowly making progress, but it may be desirable to speed this up by implementing this ourselves upstream. Once that is done, the on_slash function in orml-traits should return a DispatchResult and the loans code should propagate errors instead of logging them.