concrete error message if modules are not found

[DanSchlachter]

Not sure if this is the right approach. Even though the readme.md states to run npm ci before cds w xx I was asked by several people struggling with the cap samples who get stuck at Error: Cannot find module '@capire/bookshop/package.json'

If you are used to cap projects working without npm i and not familiar with the workspace setup, you will run into this issue and don't necessarily understand why.

[cla-assistant[bot]]

All committers have signed the CLA.

[chgeo]

Looks good for me in general. @DanSchlachter Make sure to sign the CLA.