Improved error message

If no alternatives could be found, the error message is: "Attempted to call method "getContents" on class "SebastianBergmann\Diff\Diff"." This doesn't point out any error, as it doesn't tell you that the method does not exists. This PR tries to fix this.
Yeah, you're calling a method on a class, but the method is of the class. Since the method is not defined in the class, it's an undefined method of the class (I believe..). That's why I changed it to "an undefined method of class ..."