tests: Remove test-error.c and replace with a script that uses nbdsh.
test-error.c was a strange test. It tested that ext2 filesystems
could recover from block device errors after we trigger an error and
then remove the trigger, which is kind of interesting but also
irrelevant to NBD. It also failed to test that the EIO error is
properly propagated back through libguestfs.
test-error100.sh already tested error propagation correctly.
This adds a new test that tests error triggering using nbdsh, and
removes test-error.c.