Made the requirement of a non-nil formatName more explicit.

The assertion brings the error closer to the calling code.  A `nil` `formatName` will eventually crash in `_FICAddCompletionBlockForEntity()` at `[requestDictionary setObject:formatName forKey:FICImageCacheFormatKey]`.  The assert can't go in `_FICAddCompletionBlockForEntity()` because the `NSParameterAssert` macro requires `self`.