Handle poorly formed individual sensor readings

ipmitool returns a series of multiple readings per run. Some of
these readings may be of a form that parser code cannot handle. The
desired behavior in this case is for the single reading to be
dropped and for parsing to continue. This change ensures that
behavior by continuing through the loop instead of allowing
exceptions to cause the loop to exit.

Tests have been added which verify the correct behavior.

Note that though there was an opportunity to log each skipped
reading this would lead to a very large number of log messages as
many or even most samples from ipmitool are not parsed.

Change-Id: I6a3193d5a6e12c69ca5c548e5fb58d8bc9646348
Closes-Bug: #1381600
(cherry picked from commit 8323cfe92af2ad27f6611be0a6887cac12b2c630)