Updated error messages.

hw6/task1/src/main.rs

                     Some(y) => {
                         sum = y;
                     }
-                    None => return Err("Overflow"),
+                    None => return Err("Overflow would happen in sum_strings()"),
                 }
             }
             Err(_) => return Err("Given String is not a int"),
                     Some(y) => {
                         prod = y;
                     }
-                    None => return Err("Overflow"),
+                    None => return Err("Overflow would happen in mul_strings()"),
                 }
             }
             Err(_) => return Err("Given String is not a int"),