Use setLevelDirError for level not found & parse errors

This exports the previously-internal setLevelDirError function in
FileSystemUtils and uses it for if a level is not found or there was a
parsing error. Previously, if a level failed to load in these ways, it
would take you to the error screen with no error, while printing it to
the console. But this makes it more user-friendly.

As a bonus, the text is localizable, just like the existing usage of
FILESYSTEM_setLevelDirError for if a path couldn't be mounted.

desktop_version/src/FileSystemUtils.h

@@ -60,6 +60,7 @@ void FILESYSTEM_freeEnumerate(EnumHandle* handle);
 bool FILESYSTEM_levelDirHasError(void);
 void FILESYSTEM_clearLevelDirError(void);
 const char* FILESYSTEM_getLevelDirError(void);
+void FILESYSTEM_setLevelDirError(const char* text, const char* args_index, ...);
 
 bool FILESYSTEM_openDirectoryEnabled(void);
 bool FILESYSTEM_openDirectory(const char *dname);