"unknown start of token" error message is really bad

[catamorphism]

use std::list;
?use vec;

fn main() {}

This yields:

../src/test/compile-fail/bad-token-use.rs:1:13: 1:14 error: unknown start of token: 63
../src/test/compile-fail/bad-token-use.rs:1 use std::list;
                                                         ^

This is bad in a few different ways:

1. "unknown start of token" is obscure, and why can't it print out '?' instead of 63?
2. The error message does not show the erroneous code.
3. The span is messed-up.
4. I ran into this with a file in rustc, and in that case, the error message was much worse: the span was line 1 of rustc.rc. Panic-inducing.

[catamorphism]

Non-critical for 0.6, de-milestoning

[catamorphism]

Nominating for milestone 5, production-ready.

[graydon]

perfect storm of bad parser error messages, accepting

[graydon]

accepted for production-ready milestone

[emberian]

Still relevant