Importing a module that does not exist fails hard

``` rust
import std::vec;
use std;

fn main() {
}
```

```
godot:build jdm$ x86_64-apple-darwin/stage0/bin/rustc  --target=x86_64-apple-darwin  ~/src/resolve_test.rs 
rust: upcall fail 'option none', /Users/administrator/src/rustbot/workspace-snap-stage3-i386-unknown-darwin/src/src/libcore/option.rs:34
rust: domain main @0x10202d200 root task failed
```
