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.cpp

@@ -456,7 +456,7 @@ const char* FILESYSTEM_getLevelDirError(void)
     return levelDirError;
 }
 
-static void setLevelDirError(const char* text, const char* args_index, ...)
+void FILESYSTEM_setLevelDirError(const char* text, const char* args_index, ...)
 {
     levelDirHasError = true;
 
@@ -475,7 +475,7 @@ static bool FILESYSTEM_mountAssetsFrom(const char *fname)
 
     if (real_dir == NULL)
     {
-        setLevelDirError(
+        FILESYSTEM_setLevelDirError(
             loc::gettext("Could not mount {path}: real directory doesn't exist"),
             "path:str",
             fname