Improve error reporting.

Whenever we emit an error, we should set the human readable error string
in the discovery classes.

[ChangeLog][QtBluetooth][Documentation] Fix cases where device and service
discovery classes emitted an error signal but the human readable
error string was not adjusted.

Change-Id: I9680853d17d2ee4bc1293826bb7bf56cc999e2ed
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Use QLoggingCategory to categorize the various debug output streams

Task-number: QTBUG-32253
Change-Id: I193162407d0fc7eca83689e31f03e1641a494ab0
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>
Translate so far untranslated error strings.

Change-Id: I7d91404052ce987ea700e698c024475211bad025
Reviewed-by: Fabian Bumberger <fbumberger@rim.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>