I believe that `FitsWCS(header)` cannot fail (despite the fact that I have it raise an exception if all options fail), because the final class that it tries is `AffineTransform`, and I'm pretty sure that it will always succeed, since it has default values for all of its `header.get` calls.

That said, I don't mind adding the extra except clause here to make sure that this doesn't allow an exception to pass up from `FitsWCS`.