Trouble detecting when card has been removed

`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 https://github.com/adafruit/circuitpython/issues/301, but it did not fail as expected.

Start by tracing readblock() and writeblock(), then inspect how _wait_for_ready() is used to detect card availability. Reproduce removal behavior and determine what reliable failure condition is needed; done means both operations report the removed SD card instead of appearing to succeed.

Written by the indexing model from the issue text.