Hmm, do you mean like provide a message to the developer? Or provide a message to the user?
If you mean the latter, the simple answer is no :/ I think Blockly's philosophy is that if two blocks can't connect, the user should be able to figure out why from other info (eg the color of the block, its fields, connection shapes, etc).
However, if you do want to give some more information to the user you should be able to use the
connection checker interface to do that. Eg if your type checks fail in
doTypeChecks, you could send a message to a different system (which you build) that could display a message to the user.
I hope that helps! If you have any further questions please reply :D
--Beka