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.

src/librustc_driver/lib.rs

@@ -1148,9 +1148,18 @@ pub fn diagnostics_registry() -> errors::registry::Registry {
     Registry::new(&all_errors)
 }
 
+fn get_args() -> Vec<String> {
+    env::args_os().enumerate()
+        .map(|(i, arg)| arg.into_string().unwrap_or_else(|arg| {
+             early_error(ErrorOutputType::default(),
+                         &format!("Argument {} is not valid Unicode: {:?}", i, arg))
+         }))
+        .collect()
+}
+
 pub fn main() {
     env_logger::init().unwrap();
-    let result = run(|| run_compiler(&env::args().collect::<Vec<_>>(),
+    let result = run(|| run_compiler(&get_args(),
                                      &mut RustcDefaultCalls,
                                      None,
                                      None));

src/librustdoc/lib.rs

@@ -107,12 +107,19 @@ pub fn main() {
     const STACK_SIZE: usize = 32_000_000; // 32MB
     env_logger::init().unwrap();
     let res = std::thread::Builder::new().stack_size(STACK_SIZE).spawn(move || {
-        let s = env::args().collect::<Vec<_>>();
-        main_args(&s)
+        get_args().map(|args| main_args(&args)).unwrap_or(1)
     }).unwrap().join().unwrap_or(101);
     process::exit(res as i32);
 }
 
+fn get_args() -> Option<Vec<String>> {
+    env::args_os().enumerate()
+        .map(|(i, arg)| arg.into_string().map_err(|arg| {
+             print_error(format!("Argument {} is not valid Unicode: {:?}", i, arg));
+        }).ok())
+        .collect()
+}
+
 fn stable(g: getopts::OptGroup) -> RustcOptGroup { RustcOptGroup::stable(g) }
 fn unstable(g: getopts::OptGroup) -> RustcOptGroup { RustcOptGroup::unstable(g) }