How to display “error” message

#include

using namespace std;

float a,b,c,m;

int main()
{
cin>>a;
cin>>b;
cin>>c;

m=3/(1/a+1/b+1/c);
cout<<m;

return 0;

}
This is my code and i want to display “error” message the result of the action is impossible to count.