`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.
In the future different services will write to the task log, and thus
it makes sense to move the responsibility of prepending the timestamps
to the log storage service.
A task can exist in the database but not have any log stored on disk yet.
This is now returned as `204 No Content` instead of an internal server
error.
The web interface is also adjusted to cope with this.
It returns 2048 bytes at most. It'll likely be less than that, as it will
ignore the first bytes until the very first newline (to avoid returning
cut-off lines). If the log file itself is 2048 bytes or smaller, return the
entire file.
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.
