core: Log stacktrace when VDS command execution failed

Added stacktrace to debug log when an exception is raised
during VDS command execution.

Change-Id: I67d502dd74d45cf75a4fe98c840a41df58ac8425
Bug-Url: https://bugzilla.redhat.com/948744
Signed-off-by: Martin Perina <mperina@redhat.com>