roots := proc(eqn)
  local p, r;
  begin
    p := poly(eqn, x $ INDOETS);
    r := solve(p, x);
    if nops(r) < degree(p) then
      print("Some roots could not be found.");
    end_if;
    return(r);
  end_proc:

roots(x^5 - 3*x^4 + x^2 - 1 = 0);