Pybricks Code does not validate checksum during download

[dlech]

It also seems odd that we aren't getting an error from a bad checksum since the hub is not receiving all of the data. So there could be a bug there too.

Originally posted by @dlech in #403 (comment)

[dlech]

It looks like we aren't currently validating the checksum while flashing firmware in Pybricks Code. Instead, we only validate it at the very end. If we validate it as we go, then we can fail early if a packet is dropped and it would have helped identify the cause of #403.

[dlech]

Fix is released in Pybricks Beta v1.1.0-beta.4.