Remove barely used ProtocolError type

conn.go

@@ -89,13 +89,6 @@ var ErrNoRows = errors.New("no rows in result set")
 // ErrInvalidLogLevel occurs on attempt to set an invalid log level.
 var ErrInvalidLogLevel = errors.New("invalid log level")
 
-// ProtocolError occurs when unexpected data is received from PostgreSQL
-type ProtocolError string
-
-func (e ProtocolError) Error() string {
-	return string(e)
-}
-
 // Connect establishes a connection with a PostgreSQL server with a connection string. See
 // pgconn.Connect for details.
 func Connect(ctx context.Context, connString string) (*Conn, error) {

rows.go

@@ -165,7 +165,7 @@ func (rows *connRows) nextColumn() ([]byte, *pgproto3.FieldDescription, bool) {
 		return nil, nil, false
 	}
 	if len(rows.FieldDescriptions()) <= rows.columnIdx {
-		rows.fatal(ProtocolError("No next column available"))
+		rows.fatal(errors.New("No next column available"))
 		return nil, nil, false
 	}