Also add TryFindStructureDefinition() etc

[ewoutkramer]

After doing PR review on #2778, I wondered where all the changes would go in the SDK code that is actually calling resolvers. Turns out, that most of our code does not directly call IResourceResolver members, but the derivatives, the extension methods on IResourceResolver like FindStructureDefinition. Since these did not get the TryXXXX treatment yet, there is still a lot of SDK code (e.g. the snapshot generator) that is now calling the new try methods, but hidden in these FindSSTructureDefinition methods, which just return null, like they always have.

To fully profit from these new changes, we should look at our uses of these extension methods, maybe mark them as Obsolete too and then make sure our own internal calls to IResourceResolver will exploit the new error detail information. I don't know whether this can be done after 6.0, since reporting back errors from something like the SnapshotGenerator may also require breaking changes.

So, at the least, we should do an investigation.