Log mmc and sd command failures.  Reporting of routine expected errors,
such as timeouts while probing a bus or testing for a feature, is
squelched.  Also, error reporting is limited to 5 events per second,
because when an sdcard goes bad on a low-end embedded board, flooding
the console at high speed isn't helpful.

Original logging code contributed by Michal Meloun, but then I fancied
it up with squelching and ppsratecheck.