More descriptive config errors

[yannickl88]

When installing the bundle I got the following error:

Webpack is not installed in path "".

Which tells me nothing. After some digging, it meant that that I need to configure the node_modules_path in the node config.

Can you make the config error also provide a possible solution. I.e.: did you forget to configure the node_modules_path? or something.

[linaori]

Some configuration validation can already be done like here: https://github.com/hostnet/webpack-bundle/blob/master/src/Component/Configuration/Config/OutputConfig.php#L30-L33