Trouble detecting when card has been removed

[dhalbert]

readblock() and writeblock() appear to succeed even when the SD card has been removed. It appears that _wait_for_ready() is not a good test for detection. I originally added more calls to _wait_for_ready() in various places to handle bugs like adafruit/circuitpython#301, but it did not fail as expected.

[jepler]

There does not appear to be a good strategy for dealing with this problem. If you have any ideas, code it up and send us a PR!