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.

go.mod

@@ -19,7 +19,6 @@ require (
 	github.com/coreos/go-systemd v0.0.0-20190719114852-fd7a80b32e1f
 	github.com/davecgh/go-spew v1.1.1
 	github.com/decred/dcrd/dcrec/secp256k1/v4 v4.3.0
-	github.com/go-errors/errors v1.0.1
 	github.com/go-logr/logr v1.3.0 // indirect
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/gorilla/websocket v1.5.0
@@ -48,7 +47,7 @@ require (
 	github.com/lightningnetwork/lnd/tor v1.1.6
 	github.com/ltcsuite/ltcd v0.0.0-20190101042124-f37f8bf35796
 	github.com/miekg/dns v1.1.43
-	github.com/pkg/errors v0.9.1
+	github.com/pkg/errors v0.9.1 // indirect
 	github.com/prometheus/client_golang v1.11.1
 	github.com/stretchr/testify v1.9.0
 	github.com/tv42/zbase32 v0.0.0-20160707012821-501572607d02