Updates the FieldUpdateForbiddenException message.

I would like to update the following FieldUpdateForbiddenException's message:

"field_sql_storage cannot change the schema for an existing field with data."

It would be much easier to troubleshoot (when doing a feature-revert for instance) with the field name in it, like below:

"field_sql_storage cannot change the schema for an existing field (" . $field['field_name'] . ") with data."

I will attach a patch to this issue.

Thanks.