SAGA: Add AdLib fallback for IHNM GOG

This commit adds a fallback for the GOG version of I Have No Mouth... This
version is missing the AdLib instrument definition files which are necessary
to use the Miles driver. If these files are missing, the regular AdLib driver
is used, which has built-in instrument definitions. A warning is shown so the
user knows the music is inaccurate without the missing files.

engines/saga/music.cpp

@@ -41,6 +41,8 @@
 #include "common/config-manager.h"
 #include "common/file.h"
 #include "common/substream.h"
+#include "common/translation.h"
+#include "gui/message.h"
 
 namespace Saga {
 
@@ -78,13 +80,30 @@ Music::Music(SagaEngine *vm, Audio::Mixer *mixer) : _vm(vm), _mixer(mixer), _par
 				}
 				if (Common::File::exists(opl2InstDefFilename) && Common::File::exists(opl3InstDefFilename)) {
 					_driver = (MidiDriver_Multisource *)Audio::MidiDriver_Miles_AdLib_create(opl2InstDefFilename, opl3InstDefFilename);
+					_driver->property(MidiDriver::PROP_MILES_VERSION, _vm->getGameId() == GID_ITE ?
+						Audio::MILES_VERSION_2 : Audio::MILES_VERSION_3);
 				} else {
-					error("Could not find AdLib instrument definition files %s and %s", opl2InstDefFilename, opl3InstDefFilename);
+					// WORKAROUND The GOG version of IHNM is missing the AdLib
+					// instrument definition files. In this case we fall back
+					// to the regular AdLib driver, which has a built-in set of
+					// instrument definitions.
+					// We cannot distinguish between this GOG version and the
+					// case where the user has a physical version of the game,
+					// but has forgotten to copy the instrument definition
+					// files. So we show a warning that these files are missing.
+					GUI::MessageDialog dialog(
+						Common::U32String::format(
+							_("Could not find AdLib instrument definition files\n"
+							  "%s and %s. Without these files,\n"
+							  "the music will not sound the same as the original game."),
+							opl2InstDefFilename, opl3InstDefFilename),
+						_("OK"));
+					dialog.runModal();
+
+					_driver = new MidiDriver_ADLIB_Multisource(OPL::Config::kOpl3);
 				}
 			} else {
 				_driver = new MidiDriver_ADLIB_Multisource(OPL::Config::kOpl3);
-				_driver->property(MidiDriver::PROP_MILES_VERSION, _vm->getGameId() == GID_ITE ?
-					Audio::MILES_VERSION_2 : Audio::MILES_VERSION_3);
 			}
 			break;
 		case MT_MT32: