justin: don't assume Exception has message attribute

In Python3, the Exception class does not have a message attribute.
Stringifying the Exception object results in the same thing though,
and it works in both Python 2 and 3.
service: silence unused-argument warning from pylint

The options parameter is unused, but is part of the interface, so
we don't want to remove it.
Remove unused imports
svtplay_dl: silence pylint warning when using % with log