improve the exception message when saving an array into a config string

Problem/Motivation

If you try to save an array of data into a config property, and you stupidly defined it in the schema as 'string' instead of 'sequence', you get a REALLY confusing exception that says something like:

> Property my_property.0 does not exist.

It would be much more helpful if this said something like 'Tried to save an array into a string property'.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet