multi: use the "errors" package everywhere

Replace all usages of the "github.com/go-errors/errors" and
"github.com/pkg/errors" packages with the standard lib's "errors"
package. This ensures that error wrapping and `errors.Is` checks will
work as expected.

htlcswitch/linkfailure.go

@@ -1,6 +1,6 @@
 package htlcswitch
 
-import "github.com/go-errors/errors"
+import "errors"
 
 var (
 	// ErrLinkShuttingDown signals that the link is shutting down.