Fix extension sandbox tests on master.

The master branch used `extension.id` instead of `extension.metadata['Name']`
for some of the logged messages when extensions threw exceptions. This failed
with the `SandboxExtension` used in the unit tests, since the id is assigned by
`ExtensionManager.load()`, and only for extensions with entry points.

I've manually assigned an extension ID to the `SandboxExtension`, and updated
the other sandbox log messages to be consistent.

Testing done: Ran unit tests.

Reviewed at https://reviews.reviewboard.org/r/5791/