`wasp build start`: Show warning message when no environment variables were passed

[cprecioso]

We got a user struggling with wasp build start as they were trying to pass environment variables throught the shell instead of CLI arguments: https://discord.com/channels/686873244791210014/1433736962195329124/1433736962195329124

There's no reasonable situation in which there wouldn't be any environment variables, as at a minimum we'd require the DATABASE_URL.

We could detect the case when no environment variables were passed and print a warning message telling users.

[Martinsos]

I think they also got misguided by the errors they got -> errors told them to define env vars if I am correct, so they went to do it in "usual" way, which was not the correct way.

What if we instead also "enriched"/modified those errors to give more guidance, at least in this case?