Fix control reaches end of non-void function warnings

[madmiraal]

The mingw-w64 compiler, raises warnings that control reaches end of non-void function [-Wreturn-type] in:

• core/io/resource_loader.cpp line 388, in static member function static RES ResourceLoader::_load(const String&, const String&, const String&, bool, Error*)
• core/io/resource_loader.cpp line 656, in static member function static Ref<ResourceInteractiveLoader> ResourceLoader::load_interactive(const String&, const String&, bool, Error*)
• core/os/os.cpp line 791, in member function virtual PoolStringArray OS::get_connected_midi_inputs()
• core/packed_data_container.cpp line 82, in member function Variant PackedDataContainer::_iter_get_ofs(const Variant&, uint32_t)
• core/project_settings.cpp line 1087, in member function Error ProjectSettings::save_custom(const String&, const CustomMap&, const Vector<String>&, bool)
  -platform/windows/windows_dir_access.cpp line 336, in member function virtual String WindowsDirAccess::get_filesystem_type() const

It appears that the compiler doesn't recognise that the ERR_FAIL_V and ERR_FAIL_V_MSG macros if (true) condition implies that the else branch will never be reached:

RebelEngine/core/error_macros.h

Lines 701 to 712 in 5afd8c9

	#define ERR_FAIL_V_MSG(m_retval, m_msg) \
	if (true) { \
	_err_print_error( \
	__FUNCTION__, \
	__FILE__, \
	__LINE__, \
	"Method/function failed. Returning: " _STR(m_retval), \
	m_msg \
	); \
	return m_retval; \
	} else \
	((void)0)

RebelEngine/core/error_macros.h

Lines 718 to 728 in 5afd8c9

	#define ERR_FAIL_V(m_retval) \
	if (true) { \
	_err_print_error( \
	__FUNCTION__, \
	__FILE__, \
	__LINE__, \
	"Method/function failed. Returning: " _STR(m_retval) \
	); \
	return m_retval; \
	} else \
	((void)0)

To avoid these warnings, this PR adds additional returns that mirror the if (true) returns.

[DanielaOrtner]

Thanks for the good work :)