Display all emission types in error msg if user inputs invalid option.
Display all emission types in error msg if user inputs invalid option.

before:

```
> rustc --emit foo
error: unknown emission type: `foo`
```

after:

```
> rustc --emit foo
error: unknown emission type: `foo` - expected one of: `llvm-bc`, `asm`, `llvm-ir`, `mir`, `obj`, `metadata`, `link`, `dep-info`
```

graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust