Implement error return documetation

Adds ability to add error return documetation to the binder and class reference.
Usage example:

```C++
void MyClass::_bind_method() {
	[..]
	BIND_METHOD_ERR_RETURN_DOC("load", ERR_FILE_CANT_OPEN, ERR_FILE_UNRECOGNIZED);
}
```

One function of ConfigFile was changed as example.

core/doc_data.h

@@ -67,6 +67,7 @@ public:
 		String qualifiers;
 		String description;
 		Vector<ArgumentDoc> arguments;
+		Vector<int> errors_returned;
 		bool operator<(const MethodDoc &p_method) const {
 			if (name == p_method.name) {
 				// Must be a constructor since there is no overloading.