VistAPackageInfoFetcher: improve robustness of isPatchInstalled method

Also add some extra comments.

OSEHRA-Id: http://issues.osehra.org/browse/VAF-122
Change-Id: Ie7bcf7678b68e495aeffa2455d409a9407807893
Merge "VistAPackageInfoFetcher: improve robustness of isPatchInstalled method"