localSchemaFile doesn't work if env var is present

**Intended outcome:**

If a `localSchemaFile` is defined, it will take precedent and load schema from the file, regardless of whether or not an engine schema is found.

**Actual outcome:**

the language server tries to load the schema from engine since it found an ENGINE_API_KEY. _Even if it fails_ to load the schema, the localSchemaFile won't be taken into account

**How to reproduce the issue:**

- Configure a client project to load a schema from a local schema file
- Verify the schema loads from the local schema file
- Add a .env file with an ENGINE_API_KEY (valid or not)
- Try to use CLI features

**Versions**

VSCode 1.5.2

Start at the language server's schema-loading path and trace how localSchemaFile and ENGINE_API_KEY are selected. Reproduce with a local schema, then add .env containing ENGINE_API_KEY and try the CLI features; done means the local schema is used even when the engine schema is unavailable.

Written by the indexing model from the issue text.