To prevent the need to copy the entire onion_error.go file for a new
Mission Control migration, this commit just updates the existing
lnwire21/onion_error.go file with the new CodeInvalidBlinding code. The
lnwire21 should not really ever be updated but adding a new code should
be fine as it does not affect old migrations since this is a new code.
To avoid code changing underneath the static migrations, we copy the
lnwire dependency in its current form into the migration directory.
Ideally the migrations doesn't depend on any code that might change,
this takes us a step closer.
