(cherry picked from commit a3ba91f)

Previously, any errors thrown during `didInsertElement` would leave the
running environment in an invalid state (`env.inTransaction` would be
`true` but `this._transaction` would have been nullified).

This commit ensures that we _always_ reset `inTransaction` if
`Environment.prototype.commit` is called. Thus avoiding an error RE:
"calling commit on null"...

(cherry picked from commit ab4a2e3)

(cherry picked from commit e99dd89)

`Ember.trySet` is designed to be an error-tolerant form of `Ember.set`
(per its public API documentation). However, in some circumstances an
error is still thrown when setting on a destroyed object.

Prior to this change, the following would properly prevent an error:

```js
let obj = { some: { deep : 'path', isDestroyed: true }};

Ember.trySet(obj, 'some.deep', 'stuff');
```

But the following would throw an error incorrectly:

```js
let obj = { some: 'path', isDestroyed: true };

Ember.trySet(obj, 'some', 'stuff');
```

This fixes the latter case...

(cherry picked from commit 1f4a3bc)

[ci skip]