better error message in ReadOnlySymbolTable.ValidateAccepts

[MikeStall]

In SymbolValue.Set(slot, value) , we will call ReadOnlySymbolTable.ValidateAccepts to verify a type match.

This gives cryptic errors like: System.InvalidOperationException: 'Can't change ''Products:NLPayload_37368736'' from Microsoft.PowerFx.Types.TableType to *[ID:w, Name:s, Price:w].'

Make this message easier to debug. At least include both the name and destination types (or ideally the recent field comparison checks to tell us which fields don't match)