imap: do not abort on malformed messages

Some messages contain invalid headers, textproto.ReadHeader fails with
an error:

  malformed MIME header key: From nobody Fri Jan 14 19

And all other messages of the selected folder are ignored.

Return an explicit error message to the UI and continue processing other
messages. This is consistent with what the maildir worker does.

Signed-off-by: Robin Jarry <robin@jarry.cc>

worker/imap/fetch.go

@@ -38,7 +38,13 @@ func (imapw *IMAPWorker) handleFetchMessageHeaders(
 			reader := _msg.GetBody(section)
 			textprotoHeader, err := textproto.ReadHeader(bufio.NewReader(reader))
 			if err != nil {
-				return fmt.Errorf("could not read header: %v", err)
+				imapw.worker.Logger.Printf(
+					"message %v: could not read header: %v", _msg.Uid, err)
+				imapw.worker.PostMessage(&types.Error{
+					Message: types.RespondTo(msg),
+					Error:   err,
+				}, nil)
+				return nil
 			}
 			header := &mail.Header{message.Header{textprotoHeader}}
 			imapw.worker.PostMessage(&types.MessageInfo{