Better error messages for client code gen

[sbdchd]

I saw https://github.com/encoredev/encore/issues/118 which is very similar / the same issue, but the current error message isn't great

❯ encore gen client testaug5-atf2 --lang=typescript

error: could not fetch API metadata: GET /apps/testaug5-atf2/envs/@primary/meta: http 404 Not Found: code=env_not_deployed detail=null

but this worked:

encore gen client testaug5-atf2 --lang=typescript --env=local > client.ts 

[eandre]

Thanks, you're absolutely right. Will fix this, thank you!