Return empty string instead of None (systests)

In 'execute_through_host' method return empty string if
exception is caught. This is required to avoid type errors
in tests which use that function. Also, increase log level
to 'error' for message about occurred exception.

Change-Id: I7021ee90e32f39b2183b7693d3ba6fe4c669517d
Closes-bug: #1369962
(cherry picked from commit 10195ef8622c6ff3366c679d6bcfe9939635f496)