Catch IndexError in pyls_signature_help

[mike-hearn]

Fixes #459

This is how I'm addressing the issue locally, and it appears to fix the issue and causes the signature help to work correctly in coc.nvim.

[palantirtech]

Thanks for your interest in palantir/python-language-server, @mike-hearn! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.