errors: refactor invalidArgType()

`invalidArgType()` uses `includes()` in two places where `startsWith()`
and `endsWith()` are more appropriate (at least in my opinion). Switch
to those more specific functions.

lib/internal/errors.js

@@ -309,7 +309,7 @@ function invalidArgType(name, expected, actual) {
 
   // determiner: 'must be' or 'must not be'
   let determiner;
-  if (expected.includes('not ')) {
+  if (typeof expected === 'string' && expected.startsWith('not ')) {
     determiner = 'must not be';
     expected = expected.split('not ')[1];
   } else {
@@ -320,7 +320,7 @@ function invalidArgType(name, expected, actual) {
   if (Array.isArray(name)) {
     var names = name.map((val) => `"${val}"`).join(', ');
     msg = `The ${names} arguments ${determiner} ${oneOf(expected, 'type')}`;
-  } else if (name.includes(' argument')) {
+  } else if (name.endsWith(' argument')) {
     // for the case like 'first argument'
     msg = `The ${name} ${determiner} ${oneOf(expected, 'type')}`;
   } else {