When attempting to install static media files for an extension, some

things could go wrong, and we didn't always recover gracefully. There

were a few major issues:

If there were permission issues with static media files, but we could

  write the .version file, then only the .version would be written,

  preventing future attempts at installation.
If a lock was not released, installation would stall forever.
When installation failed, the administrator would receive a pretty

  lousy and useless error message at best, and see a request never

  complete (due to a HTTP 500) at worst.
If we had a permission error getting a lock file, things would blow up

  in unhelpful ways.
If static media files were missing, pages could completely crash

  trying to locate them.

This change fully rewrites the media installation code to deal with

these issues. It now considers both a locking failure and a

permission/IO error writing a lock file to be the same problem, and will

retry. The retry count is now capped at 10 tries, so we don't spin

forever. (This will still mean up to 10 seconds of stalling, as a

worst-case scenario.)
The .version file is only written once we have successfully installed

static media, so we don't appear to succeed when we fail.
If any part of this process goes wrong, a useful error message (often

containing instructions) is sent to the user (if enabling via the

administration UI) and logged. These will also cause the extension to

disable, so it's (hopefully) not in a half-installed state.
We also now check the final result to see if we've successfully bumped

the version, and provide a suitable error if we weren't able to *due to

permission/lock issues, for instance).
When loading an extension's static bundle that we can't find (which

should be less of an issue now), we no longer dump a stack trace and

raise the exception (breaking the page). Instead, we log a more useful

"error"-level warning and just give up trying to load that static media

files. This means that extension functionality on pages may not function

correctly, but the pages will at least load without an HTTP 500.

Unit tests passed.
Manually tested with a production site. Verified that initial installation

worked, that upgrades worked, and that the right errors were provided when

permissions were bad.