ruby/metal: Emit librashader shader creation errors (#1647)

Displaying errors from librashader shader loads in Metal will make
librashader debugging easier.

ruby/video/metal/metal.cpp

@@ -162,13 +162,15 @@ struct VideoMetal : VideoDriver, Metal {
     }
 
     if(file::exists(pathname)) {
-      if (_libra.preset_create(pathname.data(), &_preset) != NULL) {
+      if (auto error = _libra.preset_create(pathname.data(), &_preset)) {
         print(string{"Metal: Failed to load shader: ", pathname, "\n"});
+        _libra.error_print(error);
         return false;
       }
 
-      if (_libra.mtl_filter_chain_create(&_preset, _commandQueue, nil, &_filterChain) != NULL) {
+      if (auto error = _libra.mtl_filter_chain_create(&_preset, _commandQueue, nil, &_filterChain)) {
         print(string{"Metal: Failed to create filter chain for: ", pathname, "\n"});
+        _libra.error_print(error);
         return false;
       };
     } else {