Issue #100: process invalid boundary box.

Stop processing if explicitly specified boundary box is invalid, even if
we can get the right one from another source.

map_machine/mapper.py

@@ -283,6 +283,8 @@ def render_map(arguments: argparse.Namespace) -> None:
 
     if arguments.boundary_box:
         boundary_box = BoundaryBox.from_text(arguments.boundary_box)
+        if not boundary_box:
+            fatal("Invalid boundary box.")
 
     elif arguments.coordinates:
         coordinates: Optional[np.ndarray] = None