Show warning, not error for unverifiable, generics-based workflows

See #33. Based on a suggestion by @cboehme.

This tweaks one of the test workflows to include a case where one command outputs `Object`, and the next requires `String`. Internally, this is based on generics and works, so we display a warning here, not an error. This commit unifies this new warning with the previous two (for missing input and output annotations).