Improve `lint` output when attempting to process something that is not a schema

This is what we get so far:

```sh
jsonschema lint schema.json
error: Could not determine the base dialect of the schema
``` 

Which is very non descriptive. We should also hint at the bottom at something like "Are you sure the input is a valid JSON Schema and its dialect is known?" and potentially also hint at `--default-dialect`/`-d`