Give better error message when manifest.toml doesn't exist

[jackaperkins]

Starting out with a default create-react-app and trying to server it via webxdc-dev run http://localhost:3000 gives me the following error Invalid manifest.toml, please check the format, and then webxdc-dev exits.

It would be more helpful if it could tell the difference between a parse error and a 404, or even just do a warning and continue as it does with serving directory of static files without a manifest.toml.

[ralphtheninja]

I looked into this and the error here should've been caught and not generated any error, however it looks like the react app replies with the content of index.html which obviously fails to parse as a valid .toml file.

[ralphtheninja]

The fix should now produce the following output: