Spec multi with gen, add test_not and fail SCTs

[machow]

• allows multi to be used with generator, e.g. Ex().multi(subtest(i) for i in range(10))
• test_not fails if all subtests pass
• fail fails all the time (I mean, unless it were put in a test_not... ;). It may be useful in the future for printing out information. address issue test_not() #34. It only looks for TestFail exceptions, so may error out sometimes due to errors raised for instructors or other SCT related errors (see issue Rethinking how pythonwhat raises errors for instructors #130).
• try to pre-empt common syntax problems and raise helpful errors (see issue Pre-empt syntax errors and raise useful warnings for instructors #162)

[filipsch]

test_not! Awesome!