#523 Somewhat unhelpful Bandpass error messages

I've added some try/except clauses as suggested, and a unit test.  Bandpass and PowerSpectrum were pretty straight-forward, since for Bandpass all wavelengths should be valid test cases, and for PowerSpectrum, k=1.0 was already being tested so I decided it wouldn't hurt to test that again.  

For SED, I decided to arbitrarily test at wavelength 700.0 nm, but then only trap NameErrors.  These are the types that show up for the cases @barnabytprowe and @rmjarvis mentioned above.  This allows things like ZeroDivisionErrors to pass, which is reasonable since these don't necessarily indicate an invalid spectrum for Bandpasses which don't cover 700 nm.

I also tweaked some error messages and minor 100-column cases.