Modify code to throw exceptions for missing files

[vaneyken]

As per discussion at pipeline meeting this morning - wherever the code currently checks for missing files or catches file loading errors (esp. when loading calibration files into an obsfile object), let's just let it bail and throw an appropriate exception (or even better, print an informative message and then throw an exception). This prevents failed loads showing up as hard-to-trace glitches further down the line; and the exceptions can always be caught and handled at a higher level if need be.

[abwalter]

If it can't load the wavecal or hotpix timemask it now throws IOErrors.