Error catching in user_categorisation.R

[RayStick]

'Categorise this data element into one or more domains, e.g. 5 or 5,8:'

No input will break the code (it accepts anything and reads it in as a string) but it would be good to catch silly input errors and feedback to the user, so they don't get saved in the csv. However, the review stage mitigates this somewhat (i.e. a user may spot an error in the review stage and correct it themselves)