In this commit, we update the tower+link logic to tag a commitment as
the new (tweakless) format if it applies. In order to do this, the
BackupTask method has gained an additional parameter to indicate the
type of commitment that we're attempting to upload. This new tweakless
bool is then threaded through all the way to back up task creation to
ensure that we make the proper input.Input.
Finally, we've added a new test case for each existing test case to test
each case w/ and w/o the tweakless modifier.
This commit introduces a new error type LinkFailureError which is used
to distinguish the different kinds of errors that we can encounter
during link operation. It encapsulates the information necessary to
decide how we should handle the error.
