Add some logging for nose_discovery process

Methods of DiscoveryPlugin were modified: logic that controls processing
of testset and test entities against db has been wrapped in try...except
block with purpose to provide logging for possible errors which may
occur during this process.

Change-Id: I7647c292ed9f8cb7018cbed64a82a0110001d3a0
Merge "Add some logging for nose_discovery process"