title: Can I capture warning messages during format conversion in pybel?
body: <p>I have a list of molecules in SMILES format that I am converting to InChIKey format. However, a number of them are generating warnings. I would like to be able to generate a file showing the warnings generated in the process of converting each molecule, so that a team member can review them. Is it possible to capture these messages programmatically, instead of just logging them?</p>
  
  <p>A snippet illustrating what I'd like to be able to do:</p>
  
  <pre><code>import pybel
  
  smiles = pybel.readfile('smi', 'smiles.txt')
  converted = []
  for mol in smiles:
      smiles_str = mol.write('smiles')
      inchikey_str = mol.write('inchikey')
      warnings = None # Is there something I can do here to capture the warnings?
      converted.append((smiles_str, inchikey_str, warnings))
  </code></pre>