Well if you are going to nitpick then you need to do this.
Since if you get another error in this function you code will
hide the error.

Code:
CREATE OR REPLACE FUNCTION isnumber(strn_len IN VARCHAR2)
   RETURN BINARY_INTEGER
IS
   data_conversion_error EXCEPTION;
   PRAGMA EXCEPTION_INIT (
     data_conversion_error, -6502 );
   testval NUMBER;
BEGIN
   testval:=strn_len;
   RETURN 1;
EXCEPTION
   WHEN data_conversion_error THEN
      RETURN 0;
   WHEN OTHERS THEN
      RAISE;
END;
/