feat(errmsg): support errors wrapped through errors.Join (#21)

Because

- When joining several errors with end-user messages through
`errors.Join`,
  `errmsg.Message` returned only the first end-user message.

This commit

- Supports multiple end-user messages joined with `errors.Join`.

errmsg/errmsg.go

@@ -3,6 +3,7 @@ package errmsg
 import (
 	"errors"
 	"fmt"
+	"strings"
 )
 
 // endUserError is an error that holds an end-user message.
@@ -18,7 +19,7 @@ func (e *endUserError) Error() string { return e.cause.Error() }
 func (e *endUserError) Unwrap() error { return e.cause }
 
 // As implements the required function to ensure errors.As can properly match
-// endUserEror targets.
+// endUserError targets.
 func (e *endUserError) As(target any) bool {
 	if tgt, ok := target.(**endUserError); ok {
 		*tgt = e
@@ -44,9 +45,23 @@ func AddMessage(err error, msg string) error {
 // Message extracts an end-user message from the error.
 func Message(err error) string {
 	for err != nil {
-		eu := new(endUserError)
-		if errors.As(err, &eu) && eu.message != "" {
-			return eu.message
+		if endUserErr, ok := err.(*endUserError); ok {
+			return endUserErr.message
+		}
+
+		// If the error was generated through errors.Join, Unwrap returns an
+		// array of errors, several of which might contain a message.
+		if joinedErr, ok := err.(interface{ Unwrap() []error }); ok {
+			unwrappedErrs := joinedErr.Unwrap()
+			msgs := make([]string, 0, len(unwrappedErrs))
+			for _, uwe := range unwrappedErrs {
+				msg := Message(uwe)
+				if msg != "" {
+					msgs = append(msgs, Message(uwe))
+				}
+			}
+
+			return strings.Join(msgs, " ")
 		}
 
 		err = errors.Unwrap(err)

errmsg/errmsg_test.go

@@ -49,17 +49,18 @@ func TestAddAndExtractMessage(t *testing.T) {
 		},
 		{
 			name:    "multi-message error",
-			wantMsg: "An error happened. Something went wrong.",
-			wantErr: "bang: boom",
-			err: AddMessage(
-				// handle error coming from downstream
-				fmt.Errorf("bang: %w",
-					// downstream error also contains message
-					AddMessage(errors.New("boom"), "Something went wrong."),
-				),
-				// add message to downstream error
-				"An error happened.",
-			),
+			wantMsg: "Please check your input. Something went wrong in condition 1. Condition 2 failed.",
+			wantErr: "checking conditions: evaluating condition 1: boom\nbang",
+			err: func() error {
+				cond2Err := AddMessage(fmt.Errorf("bang"), "Condition 2 failed.")
+				cond1Err := fmt.Errorf(
+					"evaluating condition 1: %w",
+					AddMessage(fmt.Errorf("boom"), "Something went wrong in condition 1."),
+				)
+
+				jointErr := fmt.Errorf("checking conditions: %w", errors.Join(cond1Err, cond2Err))
+				return AddMessage(jointErr, "Please check your input.")
+			}(),
 		},
 	}