Replace specific error message check with error code check (#247)

* adjusted error test

* change to use 'assert true'

Author: TheAhmir

exercises/practice/grains/run_test.v

@@ -32,23 +32,23 @@ fn test_error_0() {
 	if res := grains_on_square(0) {
 		assert false, 'invalid square number should return error'
 	} else {
-		assert err.msg() == 'square must be between 1 and 64'
+		assert true
 	}
 }
 
 fn test_error_1() {
 	if res := grains_on_square(-1) {
 		assert false, 'invalid square number should return error'
 	} else {
-		assert err.msg() == 'square must be between 1 and 64'
+		assert true
 	}
 }
 
 fn test_error_65() {
 	if res := grains_on_square(65) {
 		assert false, 'invalid square number should return error'
 	} else {
-		assert err.msg() == 'square must be between 1 and 64'
+		assert true
 	}
 }