Running an unknown command should throw an error

### Version
2.9.3

### Reproduction link
[https://codesandbox.io/s/vue](https://codesandbox.io/s/vue)

### Steps to reproduce
1. Run `vue foo`


### What is expected?
I would expect an error to be thrown.

### What is actually happening?
It does nothing and exits silently.

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FWIW this is on macOS 10.13.3.