oeqa: add proper handling for command errors where needed

oeqa: add proper handling for command errors where needed

For use outside of tests themselves, we want a better error than
AssertionError, so create one and allow us to request it when calling
runCmd(). This enables us to avoid tracebacks during master image
operations if the power control command fails.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>