Show an error message if note should be created for message without a message ID

[froilo]

Moved from https://www.froihofer.net/bugzilla/show_bug.cgi?id=73

XNote depends on the presence of a messageId for assigning notes to e-mails. Therefore, the creation of notes for messages without a messageId should be prevented. Regarding user experience it would probably be the best not to disable the items for creation of the note (button, context menu), but to show an error instead so that the reason is known. Otherwise, this might lead to bug reports asking why it's not possible to add note to certain e-mails.

It seems, messages without a messageId are a rare case and this item is therefore of low priority

[Johann-Tree]

Hint: Thunderbird creates internally a MD5-Message-ID when there is none found in the message, looking like this example: <md5:bzqPHcf+eXwbqlAr+y7xDg==> By these MD5-Message-IDs you can access these messages.