Raise ValueError if you try to parse an invalid SMILES string.

RDKit will print something out to STDErr explaining the mistake.
Make exception for handling helium in RDKit.
Check that RDKit was installed with InChI support.
Editing the installation instructions to include rdkit.

These are not well tested, especially the linux and windows ones,
but it's a start.
More detailed linux installation instructions for rdkit.