blocks: no warnings when connecting 'incompatible' blocks

(This one probably isn't trivial, as I think it's partially a blockly/Python limitation)

One of the things that blocks is quite good at is avoid syntax errors.

However, our current implementation doesn't protect the user from type error as well as it could.

For example, the following code fails as soon as it is run on the micro:bit

PXT gives slightly more useful feedback:

Do we have a way of giving more immediate feedback to users about type mismatches? Or even better, perhaps (although it's spooky action at a distance) automatically inserting the required conversion so it 'just works'?

The issue names no files, tests, or entry points. Start by reproducing the incompatible-block example in the browser and on the micro:bit, then investigate the current Blockly/Python type-handling behavior. Done would require an agreed approach that gives immediate type-mismatch feedback or inserts safe conversions.

Written by the indexing model from the issue text.