LibKeyboard: Change some Optional<T> returns to ErrorOr<T>

Makes CharacterMapFile::load_from_file and CharacterMap::load_from_file
return ErrorOr instead of Optional. This makes them a little nicer to
use and a little easier to read, as they seem to have been approximating
this.

Userland/Libraries/LibKeyboard/CharacterMap.h

@@ -19,7 +19,7 @@ class CharacterMap {
 
 public:
     CharacterMap(const String& map_name, const CharacterMapData& map_data);
-    static Optional<CharacterMap> load_from_file(const String& filename);
+    static ErrorOr<CharacterMap> load_from_file(const String& filename);
 
 #ifndef KERNEL
     int set_system_map();