Fix message of alert on Backwards incompatible schema when data type has been changed

[RamanDamayeu]

If we change data type of a column with get Backwards incompatible schema change.

When we compare revisions we will see that data types have been changed.

In the same type Alert (notification) would still have a confusing message with "Missing field: [field_name]".

Seems, it's better to show that the data type of the field has been changed from one value to another.