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'?

該 issue 未指出任何檔案、測試或進入點。首先在瀏覽器和 micro:bit 上重現不相容 block 的範例，然後調查 Blockly/Python 目前的型別處理行為。完成的標準是形成一個經過同意的方法，能夠立即提供型別不相符的回饋，或插入安全的轉換。

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