Print exceptions from scenarios only in case of debug

Exceptions are standard way to stop exectuion of one scenario iteration
so we shouldn't print them to end user, because it is "OK" that there are
exceptions specially in case of big load

Print ITER START/END and result of scenario loop (success/error)

Change-Id: I32a69b5b274097b5b06479a48f22ea8633d5cadd
Closes-bug: #1282088