Make error message for mismatched string children more informative

[maier49]

Enhancement

Even though the actual and expected values for a string are passed to the AssertionError's constructor, the resulting error message doesn't actually give any hint as to what was wrong. Here is an example error message when the actual node had 'foo' as a child, and the expected node had 'bar':

AssertionError: Render unexpected
  at <_build\src\support\assertRender.js:141:17>
  at Array.forEach  <native>
  at assertChildren  <_build\src\support\assertRender.js:138:20>
  at Object.assertRender [as default]  <_build\src\support\assertRender.js:202:21>
  at Test.string children unequal [as test]  <tests\unit\support\assertRender.ts:126:4>
  at <node_modules\intern\lib\Test.js:191:24>
  at <node_modules\intern\browser_modules\dojo\Promise.ts:393:15>
  at runCallbacks  <node_modules\intern\browser_modules\dojo\Promise.ts:11:11>
  at <node_modules\intern\browser_modules\dojo\Promise.ts:317:4>
  at run  <node_modules\intern\browser_modules\dojo\Promise.ts:237:7>