dm-flakey: Clean up parsing messages

There were a number of cases where the error message for an invalid
table line did not match the actual problem. Fix these. Additionally,
error out when duplicate corrupt_bio_byte, random_read_corrupt, or
random_write_corrupt features are present. Also, error_reads is
incompatible with random_read_corrupt and corrupt_bio_byte with the READ
flag set, so disallow that.

Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>