[objectivec] Fix memory leak of exceptions raised by RaiseException() (#4556)

* Fix memory leak of exceptions raised by RaiseException()

Currently exceptions raised by RaiseException() is never deallocated because:

* ARC is disabled for this library: https://github.com/google/protobuf/blob/master/BUILD#L913
* It is constructed with `+alloc` but is never `-release`d.

This change fixes the issue by using `-[NSException exceptionWithName:...]` instead, which returns an autoreleased instance, so it is deallocated properly.

* Fix format.