add SQLOFunc type and update SQLO error messages

dbx.go

@@ -10,6 +10,9 @@ import (
 // ModuleDBName is the name of the database module.
 const ModuleDBName = "database"
 
+// SQLOFunc is a function that returns a *sql.DB pointer.
+type SQLOFunc = func() *sql.DB
+
 // Queryable interface is an SQL driver object that can execute SQL statements
 // for Jet.
 type Queryable interface {