stbt.as_precondition: preserve screenshot from intercepted failures
stbt.as_precondition: preserve screenshot from intercepted failures

Just because the error comes from inside the `as_precondition` context,
to be marked as a "less interesting" failure, doesn't mean that there's
no value in keeping the screenshot from the intercepted Exception.

pylint W0201 is "attribute-defined-outside-init".

stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,martynjarvis/stb-tester,stb-tester/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,wmanley/stb-tester,martynjarvis/stb-tester,martynjarvis/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester