Allow Cell::render() method to work with relative directories.

Refs #3945
Raise a cell specific error when views are missing.

The standard MissingView error page references controllers. By using
a more specific error message we can avoid referencing the controller
and only mention that the cell view is missing.
Add custom error page for missing cell errors.

Refs #3945
Empty the buffered content stack when rendering error pages.

When rendering an error page any buffered content should be discarded.
This fixes funny layout issues when cell errors are thrown, or when
missing element errors occur.
Fix incorrect @since tag.
Remove 'cake_dev' domain.
Remove more cake_dev domain references.
Remove output buffer cleaning as it causes all the tests to fail.
Fix PHPCS errors.
Merge pull request #4070 from cakephp/issue-3945-cell-error

Improve errors for cells

Fixes #3945