Confusing CLI error message when mixing --language and :Lang suffixes

If you use both `--language` and a `:Lang` suffix on a path, you get a confusing error message:

```
λ :!semantic graph --language=Haskell ~/Desktop/test.md:Haskell
semantic: /Users/rob/Desktop/test.md:Haskell: openBinaryFile: does not exist (No such file or directory)
```

We should probably tell you that you can’t mix these formats instead.