improve error message for empty SDL files

[EverlastingBugstopper]

current error message:

$ rover subgraph publish my-federated-graph@my-variant --schema schema.graphql --name empty --routing-url http://127.0.0.1/

error: Could not read SDL from schema.graphql

Caused by:
    '' was empty

proposed better error message

$ rover subgraph publish my-federated-graph@my-variant --schema schema.graphql --name empty --routing-url http://127.0.0.1/

error[E0XX]: Invalid schema: 'schema.graphql' is an empty file.
  Make sure your schema file is populated with the subgraph schema you are trying to publish.