Wrap a native call that may fail on alternative runtimes

Native calls like this prevent ADAL being run on alternative runtimes (e.g. Mono) - so catch the EntryPointNotFoundException and return an appropriate value.
"Unknown" is an acceptable response when this call succeeds, so it should be an acceptable response when the call fails too.
Merge pull request #125 from TheMysteriousX/master

Wrap a native call that may fail on alternative runtimes