Add pertinent error message if no options and/or values are being supplied for a protocol parameter in an Assay workflow

[Zigur]

This is part of PR #373

The error message must be something along the lines of "No IRI supplied for ontology annotation {annotation_term}. Please provide a valid IRI or use a string instead of the ontology annotation."

The error actually seems to be related to the lack of selected values for some options (i.e. parameter values or characteristic values) in the workflow. The workflow should be validated beforehand and a meaningful error message should be sent back.

[Zigur]

The behaviour was correct. As one of the protocol parameters had no selected values, the resulting assay was empty. Ideally, we should check that all parameters have at least one selected value and return an error message if not