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.

There are probably lots of other places that this happens, but this is the one that I'm hitting when trying to run ADAL on a headless *NIX system.
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