Any error message for which you know the number can be converted to a named exception in your code, eg
SQL> declare
  2    x xmltype;
  3  begin
  4      x := xmltype('some junk');
  5  end;
  6  /
declare
*
ERROR at line 1:
ORA-31011: XML parsing failed
ORA-19213: error occurred in XML processing at lines 1
LPX-00210: expected '<' instead of 's'
ORA-06512: at "SYS.XMLTYPE", line 310
ORA-06512: at line 4
SQL> set serverout on
SQL> declare
  2    x xmltype;
  3    invalid_xml exception;
  4    pragma exception_init(invalid_xml,-31011);
  5  begin
  6      x := xmltype('some junk');
  7  exception
  8    when invalid_xml then
  9       dbms_output.put_line('This did not work');
 10  end;
 11  /
This did not work
PL/SQL procedure successfully completed.