Force a 404 code for tiles out of bounds (#2365)

Author: doublebyte1

pygeoapi/provider/mvt_proxy.py

@@ -176,7 +176,7 @@ def get_tiles(self, layer=None, tileset=None,
                     else:
                         resp = session.get(f'{base_url}/{layer}/{z}/{x}/{y}{url_query}')  # noqa
 
-                    if resp.status_code == 404:
+                    if resp.status_code in [404, 500]:
                         if (self.is_in_limits(self.get_tilematrixset(tileset), z, x, y)): # noqa
                             return None
                         raise ProviderTileNotFoundError