logilab-common #84159 lgc.testlib uses sys.exit [validation pending]

lgc.testlib uses sys.exit when unittest2 is not found. This is evil and an ImportError must be raised instead.

priorityimportant
typebug
done in0.57.2
load0.500
load left0.000
closed by#d3c1fa91e736 replace sys.exit with raise ImportError (closes: #84159)