Fix most linter warnings reported by 'staticcheck'. This doesn't fix all
of them, some unused functions are still there, and some generated code
also still triggers some warnings. Most issues are fixed, though.
No functional changes, except for the captialisation of some error
messages.
Back in the days when I wrote the code, I didn't know about the
`require` package yet. Using `require.NoError()` makes the test code
more straight-forward.
No functional changes, except that when tests fail, they now fail
without panicking.
`os.IsNotExist()` is from before `errors.Is()` existed. The latter is the
recommended approach, as it also recognised wrapped errors.
No functional changes, except for recognising more cases of "does not
exist" errors as such.
The add-on code was copy-pasted from other addons and used the GPL v2
license, whereas by accident the LICENSE text file had the GNU "Affero" GPL
license v3 (instead of regular GPL v3).
This is now all streamlined, and all code is licensed as "GPL v3 or later".
Furthermore, the code comments just show a SPDX License Identifier
instead of an entire license block.
