Better error when Scan fails due to unknown destination type

fixes #31

query.go

@@ -246,7 +246,7 @@ func (rows *Rows) Scan(dest ...interface{}) (err error) {
 				rows.Fatal(err)
 			}
 		default:
-			rows.Fatal(errors.New("Unknown type"))
+			rows.Fatal(fmt.Errorf("Scan cannot decode into %T", d))
 		}
 
 		if vr.Err() != nil {

query_test.go

@@ -439,6 +439,7 @@ func TestQueryRowErrors(t *testing.T) {
 
 	type allTypes struct {
 		i16 int16
+		i   int
 		s   string
 	}
 
@@ -455,6 +456,7 @@ func TestQueryRowErrors(t *testing.T) {
 		{"SYNTAX ERROR", []interface{}{}, []interface{}{&actual.i16}, "SQLSTATE 42601"},
 		{"select $1::text", []interface{}{"Jack"}, []interface{}{&actual.i16}, "Cannot decode oid 25 into int16"},
 		{"select $1::int8range", []interface{}{int(705)}, []interface{}{&actual.s}, "Cannot encode int into oid 3926 - int must implement Encoder or be converted to a string"},
+		{"select 42::int4", []interface{}{}, []interface{}{&actual.i}, "Scan cannot decode into *int"},
 	}
 
 	for i, tt := range tests {