Marshal error with full context chain

When using the new `Augment` function to add context and wrap errors,
the error message in Go is chained and displayed with the full chain
of context:

```
err := InternalService("code", "message", nil)
augmentedErr := Augment(err, "context", nil)

// This is "internal_service.code: context: message"
augmentedErr.Error()
```

But the marshaled error only displays the first level of context,
i.e.

```
// This is just "context"
Marshal(augmentedErr.(*Error)).Message
```

This commit changes the Marshal behaviour to include the full context
chain of error, so it's actually useful outside of Go when marshaled.

```
// This is now "context: message"
Marshal(augmentedErr.(*Error)).Message
```

marshaling.go

@@ -1,6 +1,8 @@
 package terrors
 
 import (
+	"strings"
+
 	pe "github.com/monzo/terrors/proto"
 	"github.com/monzo/terrors/stack"
 )
@@ -15,22 +17,38 @@ func Marshal(e *Error) *pe.Error {
 		}
 	}
 
+	// Build message with all the context
+	errMessage := strings.Builder{}
+	errMessage.WriteString(e.Message)
+	next := e.cause
+	for next != nil {
+		errMessage.WriteString(": ")
+		switch typed := next.(type) {
+		case *Error:
+			errMessage.WriteString(typed.Message)
+			next = typed.cause
+		case error:
+			errMessage.WriteString(typed.Error())
+			next = nil
+		}
+	}
+
 	retryable := &pe.BoolValue{}
 	if e.IsRetryable != nil {
 		retryable.Value = *e.IsRetryable
 	}
 
-	err := &pe.Error{
+	err := pe.Error{
 		Code:      e.Code,
-		Message:   e.Message,
+		Message:   errMessage.String(),
 		Stack:     stackToProto(e.StackFrames),
 		Params:    e.Params,
 		Retryable: retryable,
 	}
 	if err.Code == "" {
 		err.Code = ErrUnknown
 	}
-	return err
+	return &err
 }
 
 // Unmarshal a protobuf error into a local error

marshaling_test.go

@@ -1,6 +1,7 @@
 package terrors
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -122,6 +123,61 @@ var marshalTestCases = []struct {
 			Retryable: nil,
 		},
 	},
+	// Wrapped errors
+	{
+		Augment(&Error{
+			Code:    ErrInternalService,
+			Message: "bar",
+		}, "foo", nil).(*Error),
+		&pe.Error{
+			Code:      ErrInternalService,
+			Message:   "foo: bar",
+			Retryable: nil,
+			Params:    map[string]string{},
+		},
+	},
+	{
+		Augment(&Error{
+			Code:    ErrInternalService,
+			Message: "bar",
+		}, "foo", map[string]string{"key": "value"}).(*Error),
+		&pe.Error{
+			Code:      ErrInternalService,
+			Message:   "foo: bar",
+			Retryable: nil,
+			Params:    map[string]string{"key": "value"},
+		},
+	},
+	{
+		// Nested Augment
+		Augment(
+			Augment(&Error{
+				Code:    ErrInternalService,
+				Message: "baz",
+			},
+				"bar",
+				map[string]string{"key": "value"},
+			),
+			"foo",
+			map[string]string{"key2": "value2"},
+		).(*Error),
+		&pe.Error{
+			Code:      ErrInternalService,
+			Message:   "foo: bar: baz",
+			Retryable: nil,
+			Params:    map[string]string{"key": "value", "key2": "value2"},
+		},
+	},
+	{
+		// Wrapping a Go error
+		Augment(fmt.Errorf("a go error"), "boom", map[string]string{"key": "value"}).(*Error),
+		&pe.Error{
+			Code:      ErrInternalService,
+			Message:   "boom: a go error",
+			Retryable: nil,
+			Params:    map[string]string{"key": "value"},
+		},
+	},
 }
 
 func TestMarshal(t *testing.T) {
@@ -235,6 +291,42 @@ var unmarshalTestCases = []struct {
 			Retryable: nil,
 		},
 	},
+	// Wrapped errors only gets unmarshaled as a single error
+	{
+		&Error{
+			Code:    ErrInternalService,
+			Message: "foo: bar: baz", // Augment(Augment(bazErr, "bar", nil), "foo", nil)
+			Params:  map[string]string{},
+		},
+		&pe.Error{
+			Code:    ErrInternalService,
+			Message: "foo: bar: baz",
+			Retryable: &pe.BoolValue{
+				Value: false,
+			},
+		},
+	},
+	{
+		&Error{
+			Code:    ErrInternalService,
+			Message: "foo: bar",
+			Params: map[string]string{
+				"key":  "value",
+				"key2": "value2",
+			},
+		},
+		&pe.Error{
+			Code:    ErrInternalService,
+			Message: "foo: bar",
+			Retryable: &pe.BoolValue{
+				Value: false,
+			},
+			Params: map[string]string{
+				"key":  "value",
+				"key2": "value2",
+			},
+		},
+	},
 }
 
 func TestUnmarshal(t *testing.T) {