Extensions could end up in a half-loaded state if something went wrong

(such as a static media permission issue) during extension

initialization. They'd be removed from the list of initialized

extensions, but by this time they'd have instantiated and likely

registered hooks. This meant that subsequent attempts would make further

attempts to set up those hooks, causing more and more entries to pile

up. This could lead to the server getting swamped, and could cause all

sorts of page breakages, or other kinds of errors.
The extension manager now takes a more thorough approach to

uninitializing extensions when they fail. It performs a full

_uninit_extension on them, which itself gracefully handles any issues

that might come up while attempting to uninitialize. This ensures that

every part of the extension we have any control over will be removed

from the extension manager and from memory.

Unit tests pass.