Give a nicer error for non-Unicode arguments to rustc and rustdoc

Previously, any non-Unicode argument would panic rustc:

```
$ rustc $'foo\x80bar'
error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report:
https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

thread 'rustc' panicked at 'called `Result::unwrap()` on an `Err` value:
"foo�bar"', /checkout/src/libcore/result.rs:859 note: Run with
`RUST_BACKTRACE=1` for a backtrace.
```

Now it gives a clean error:

```
$ rustc $'foo\x80bar'
error: Argument 1 is not valid Unicode: "foo�bar"
```

Maybe fixes #15890, although we still can't *compile* arbitrary file names.

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