chanbackup: continue recovery if channel already exists

[guggero]

While recovering a multi backup file, if for some reason one of the channels in that backup already exists, the whole recovery is aborted.
This leaves the restore in a half-finished state and a re-try or resume is not possible.
With this small fix, the same multi-file can be added again and all channels that were not imported before can now be added.

[guggero]

@Roasbeef bump as potential candidate for 0.8.2.

[Roasbeef]

Sound change. I've also seen instances in the past where a user hits CRTL-C early because things are taking too long, but then can't restart the process easily.

Small diff, so no major comments, but it would be nice to have a test for this either at the unit test, or integration test level, to ensure we're checking the proper error (though at a cursory scan, it looks correct to me).

[guggero]

Added a test on the itest level to make sure an SCB file can be imported twice without running into an error.
Unit test would be quite a bigger diff since the *channeldb.DB and *contractcourt.ChainArbitrator of chanDBRestorer would need to be abstracted behind an interface to be able to mock them.

[cfromknecht]

LGTM 🔥

[Roasbeef]

LGTM 🍁