Removed Exception logic when creating relationships

Replaced exception handling in `MR_setRelationships:forKeysWithObjects:withBlock:` by checking for the correct type before accessing values.

According to [Apple](https://developer.apple.com/library/ios/documentation/cocoa/conceptual/ProgrammingWithObjectiveC/ErrorHandling/ErrorHandling.html)
> You should not use a try-catch block in place of standard programming checks for Objective-C methods.