A layout extending a missing layout throws a missing-layout exception
A view extendinga missing view throws a missing-view exception
Now, an element extending a missing element throws a logic exception
in addition "absolute" paths can be used such that (using elements as an
example)
$this->extend('foo') - extends View/Elements/foo.ctp
$this->extend('/foo') - extends View/foo.ctp
Closes#2504