handle missing JS files with a console log
Use console.error
Added stylesheet warning for missing css asset
Merge pull request #1424 from odoo-dev/saas-5-handlemissingjs-nse

[FIX] Do not raise 500 error when an asset is missing

Note: those features are already present in master but could not be backported because the asset bundles are now completely different since the sass and less support where added.

If a forward porting occurs, you can drop this merge and keep master's version