The metal runtime swallows some errors

[abadams]

The metal runtime discovers some classes of error via the command_buffer_completed_handler, but this doesn't surface the error to, e.g. the next halide_device_sync call.

[abadams]

(Discovered in #7770)

[shoaibkamil]

Some helpful info: https://stackoverflow.com/questions/17813870/how-does-a-block-capture-the-variables-outside-of-its-enclosing-scope

[abadams]

This can cause schedules that produce incorrect output instead of throwing an error, so I'm adding the bug tag.

[shoaibkamil]

Fixed by #8240