#3 Input codec error handling

[Alexander Koller]

We need a general strategy for handling errors in the
input codecs -- in particular: (a) parsing errors; (b)
application of Holesem/MRS input codecs to non-nets.
Presumably the best way would be to throw an exception
that is caught in utool.cc.

[Alexander Koller]

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Allow input codecs to throw three exceptions:
- FileNotFoundException(filename)
- ParseErrorException(flex/bison error message)
- CannotConvertException(bitstring of violated conditions)