Support for logging NSErrors and similar 'out' parameters

The Cocoa error handling conventions involve passing an `NSError` error object by reference, and if the called function/method fails, the `error` parameter will have been populated with a value containing detailed information. There may be other APIs using a similar pattern; I'm implementing a C API right now that works similarly, with an `Error` struct.

It would be very useful for Catch to be able to log the error when a test assertion fails. That is, if I have the following test:

``` objc
NSError *error;
REQUIRE([[NSFileManager defaultManager] deleteItemAtPath: path error: &error]);
```

and the test fails, I'd like Catch to log the value of `error`.

Clearly this will require some syntactic changes, since the REQUIRE macro has no knowledge of this `error` variable. It might need a different macro, or something wrapped around the `&error` expression, or whatever. I spent about an hour this morning experimenting and trying to figure something out, but couldn't get it to work. I was using `INFO("Error is "...)` to log the error data, but no matter what I do either the INFO gets called before the method is called, or it goes out of scope before the failure-handling code runs, or it gets called after the failure-handling code...

Any idea how to do this? I've started digging into the Catch source, but it's pretty thorny and honestly somewhat beyond my level of C++ competence.