Add user message for uppercase RAP

rpcs3/Crypto/unself.cpp

@@ -1300,7 +1300,7 @@ bool SELFDecrypter::GetKeyFromRap(u8* content_id, u8* npdrm_key)
 
 	if (!rap_file)
 	{
-		LOG_FATAL(LOADER, "Failed to load RAP file: %s", rap_path);
+		LOG_FATAL(LOADER, "Failed to load RAP file: %s. Make sure the file extension is lowercase.", rap_path);
 		return false;
 	}
 
@@ -1457,4 +1457,4 @@ std::array<u8, 0x10> get_default_self_klic()
 	std::array<u8, 0x10> key;
 	std::copy(std::begin(NP_KLIC_FREE), std::end(NP_KLIC_FREE), std::begin(key));
 	return key;
-}
+}