fix: add missing env value; improve docs and error message (#638)

.changeset/hot-windows-watch.md

packages/create-llama/create-app.ts

packages/create-llama/helpers/env-variables.ts

packages/create-llama/index.ts

...e-llama/templates/types/simple/fastapi/README-template.md

packages/create-llama/templates/types/simple/fastapi/main.py

...lama/templates/types/streaming/fastapi/README-template.md

...es/create-llama/templates/types/streaming/fastapi/main.py