Improve the extensions page when things go wrong.
Previously, the extensions page didn't handle errors all that well. An

extension whose module couldn't even be loaded (due to an import or

syntax error) would end up causing an HTTP 500 when accessing the

extensions list resource. Trying to enable an extension that had an

error would show an alert with just the exception message and nothing

else of value.
This does a fairly large overhaul to the whole thing by storing load

error information and making them available through the API, both in

resource payloads and error payloads. It also sanitizes the fields in

the resource payloads in order to not crash.
Now, the extensions page will show a traceback along with the extension.

This is shown for any broken extensions on page load, or when attempting

to enable a broken extension.
An extension that has a failure simply in initialization (rather than a

module error) will provide the option to enable the extension, but those

with a module error will not. This is because a full reload is needed to

try loading that module again. To solve this case, a "Reload" button

will be provided any time an error is shown, which will trigger a full

extension manager reload. This all makes it much easier to develop

extensions and see what's wrong.
Trying to enable a broken extension also no longer records it as being

enabled. It will stay disabled unless it enables successfully.
While here, I did some bits of cleanup to the JavaScript and to the

display. Since we now have reload functionality, I added a link to

trigger a reload in order to find newly installed extensions. I also

added some color-based hints to show the state of an extension (enabled,

disabled, error).

Tested loading the extensions page with extensions broken both due to module errors, and due to errors in __init__. Saw that I could try enabling again only if it was broken in during initialization.


Tested enabling an extension that was broken (both ways). Same as above with the enable link.


Tested the reload functionality, both on an extension's error actions and on the "Scan for new extensions" link at the top.


Tested that disabling a broken extension properly disabled it, instead of staying enabled like before.


Tested enabling a formerly broken extension after fixing it. It properly went into an enabled state.


Tested enabling a broken extension, changing the breakage, and clicking Enable again. The error information updated to show the new breakage.


Tested that non-broken extensions properly enabled and disabled as before.


Tested that the color code on the side always reflected the proper state.


Unit tests pass.