Fix failure handling in prechecks

In a HAProxy precheck task, the command module is used, and the results
inspected by checking the stdout attribute of the result. However, if
the command fails (non-zero exit code), in some cases there may not be a
stdout attribute in the result object. This causes an AttributeError and
prevents ansible's useful diagnostic output from being displayed.

Change-Id: Id502b5d0b71fe2150a29df43154c925dca96ef06
Trivial Fix