Don't show duplicate lines (per default) such as
sr: resource: Failed to locate 'saleae-logic16-fx2.fw'.
sr: resource: Failed to open resource 'saleae-logic16-fx2.fw'.
The first one is now an sr_dbg() instead of sr_err().
Also, mention that a higher loglevel will give more information as to
where the backend is looking for resources / firmware files.
This fixes bug #806.