Improve diagnostic messages for errors in elaborating record projections

[Kmeakin]

Distinguish between the three ways that RecordProj can be an error:

• The head expression is not a record (eg Type.foo)
• The head expression is a record, but it is empty (eg {}.foo)
• The head expression is a non-empty record, but the field is missing (eg {x=foo}.foo)

I thought it was worth adding a separate diagnostic for accessing a field on a unit record, since a message about a missing field when there are no fields to choose from could be confusing