Capture std::bad_alloc in get_map_contours_mesh

[FilomenoSanchez]

Currently drawing maps with large radius or very low contour level causes the app to crash due to memory limitations. Ideally we would capture this error and send back a status code to indicate this problem. Then the UI would take care of informing the user. More importantly we need to catch the error before libcoot crashes. Otherwise after this happens further calls to molecules_container are completely ignored until a new molecules_container is created. A way to achieve this would be to have a method to set a memory limit, for example molecules_container.set_max_memory(). By default there is no limit which I imagine will be most use cases, but within Moorhen after creating a molecules_container we could use this method to set the current wasm memory limit of 2147483648 bytes. Then in molecules_container.get_map_contours_mesh there can be a check to make sure that the memory usage won't go over that limit. Just an idea, I'm not sure if that's even technically possible on your side.