Improve [Imap] [Client] Error Handling

- [+] feat(config.go): add error variables for client connection and body reading failures
- [+] refactor(export_messages.go, list_mailboxes.go, list_messages.go): replace error messages with new error variables

client/config.go

@@ -5,7 +5,11 @@
 
 package client
 
-import "github.com/emersion/go-imap/client"
+import (
+	"errors"
+
+	"github.com/emersion/go-imap/client"
+)
 
 // Config holds the configuration for the IMAP client
 type Config struct {
@@ -35,3 +39,11 @@ type IMAPClient struct {
 	config *Config
 	client *client.Client
 }
+
+var (
+	// ErrorsClientIsNotConnected is returned when an operation is attempted on a client that is not connected.
+	ErrorsClientIsNotConnected = errors.New("client is not connected")
+
+	// ErrorsFailedToReadBody is returned when there is a failure in reading the body of a message.
+	ErrorsFailedToReadBody = errors.New("failed to read body")
+)

client/export_messages.go

@@ -15,7 +15,7 @@ import (
 // ExportMessagesTo uses the provided exporter to write messages to the writer
 func (c *IMAPClient) ExportMessagesTo(writer io.Writer, exporter export.Exporter, mailbox string, numMessages uint32, config MessageConfig) error {
 	if c.client == nil {
-		return fmt.Errorf("client is not connected")
+		return ErrorsClientIsNotConnected
 	}
 
 	// Fetch messages based on the provided MessageConfig and number of messages

client/list_mailboxes.go

@@ -14,7 +14,7 @@ import (
 // ListMailboxes retrieves all available mailboxes for the connected user
 func (c *IMAPClient) ListMailboxes() ([]string, error) {
 	if c.client == nil {
-		return nil, fmt.Errorf("client is not connected")
+		return nil, ErrorsClientIsNotConnected
 	}
 
 	mailboxes := make(chan *imap.MailboxInfo, 10)

client/list_messages.go

@@ -41,7 +41,7 @@ const (
 // ListMessages lists the messages in the specified mailbox based on the MessageConfig
 func (c *IMAPClient) ListMessages(mailbox string, numMessages uint32, config MessageConfig) ([]map[string]any, error) {
 	if c.client == nil {
-		return nil, fmt.Errorf("client is not connected")
+		return nil, ErrorsClientIsNotConnected
 	}
 
 	mbox, err := c.selectMailbox(mailbox)
@@ -194,7 +194,7 @@ func (c *IMAPClient) extractBody(body map[*imap.BodySectionName]imap.Literal) (s
 			return result, nil
 		}
 		bytebufferpool.Put(buf)
-		return "", fmt.Errorf("failed to read body")
+		return "", ErrorsFailedToReadBody
 	}
 	return "", nil
 }