multi: surface server swap initiation grpc error codes

Formatting our error was stifling any grpc error returned by the
server. Instead, we bubble up our grpc error, setting an unknown
code if the server did not specifically return an error code.

client_test.go

@@ -17,6 +17,8 @@ import (
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 var (
@@ -372,3 +374,39 @@ func testSuccess(ctx *testContext, amt btcutil.Amount, hash lntypes.Hash,
 
 	ctx.finish()
 }
+
+// TestWrapGrpcError tests grpc error wrapping in the case where a grpc error
+// code is present, and when it is absent.
+func TestWrapGrpcError(t *testing.T) {
+	tests := []struct {
+		name         string
+		original     error
+		expectedCode codes.Code
+	}{
+		{
+			name: "out of range error",
+			original: status.Error(
+				codes.OutOfRange, "err string",
+			),
+			expectedCode: codes.OutOfRange,
+		},
+		{
+			name:         "no grpc code",
+			original:     errors.New("no error code"),
+			expectedCode: codes.Unknown,
+		},
+	}
+
+	for _, testCase := range tests {
+		testCase := testCase
+
+		t.Run(testCase.name, func(t *testing.T) {
+			err := wrapGrpcError("", testCase.original)
+			require.Error(t, err, "test only expects errors")
+
+			status, ok := status.FromError(err)
+			require.True(t, ok, "test expects grpc code")
+			require.Equal(t, testCase.expectedCode, status.Code())
+		})
+	}
+}