Better error message for field storage changes

Right now when you run a config import and have a storage conflict you get an error that says

"Cannot change the field type for an existing field storage."

While this might be clear when working on one field at a time, it is not specific enough when working with many fields. I propose changing the error message to say something like

"The field storage forcurrent_field_name is already set to current_field_type. Cannot change the field type for an existing field storage"

to make development easier.