Communicate errors and warnings with user

[nimobeeren]

Many unified plugins produce messages that may be useful to the user. We can use the same system to send messages from our own custom plugins. These messages can be communicated to the user in a separate window and/or directly in the editor (e.g. squiggly lines).

Some examples of things that could be done:

• invaild commands using :command
• incorrect use of :id (empty ID value, or ID does not apply to anything)
• warn when some HTML is not allowed as per sanitization rules