SaltBuilder._initPatterns incorrectly handles SMARTS errors

[adalke]

SaltBuilder._initPatterns does the following:

try:
    salt = Chem.MolFromSmarts(splitL[0])
except:
    import traceback
    traceback.print_exc(line)
    raise ValueError(line)

However, MolFromSmarts returns None on error. It doesn't raise an exception. As a result, self.salts gets a None in it, leaving StripMol() to raise a Boost ArgumentError when DeleteSubstructs is passed a None instead of a pattern.

The fix is to replace the above with:

salt = Chem.MolFromSmarts(splitL[0])
if salt is None:
    raise ValueError(line)