Empty Etox tables

[andschar]

The following call returns an error because the table doesn't contain any data:

etox_basic(99015)

https://webetox.uba.de/webETOX/public/basics/stoff.do?language=en&id=99015
The code breaks here.
Maybe we could add something like

if (inherits(tabs, 'try-error')) {
  message('ID not found! Returning NA.\n')
  return(data.frame(NA))
}

Not sure about it though.

[stitam]

I think this could work. However, this topic is related to issue #224, and PR #225. An invalid input to the etox functions should return a structure that allows subsequent vectorised extraction of elements, e.g. cas numbers in this case, so return(data.frame(NA)) might not be sufficient.

[andschar]

Do we already know how many extractor functions we want to have? Currently there are cas, inchikey and smiles.

[stitam]

I think the more the better:) I am currently working on some extractors for alanwood and chebi (only cas, inchikey, smiles)