Poor error message when attempting to use runtime from `#[start]` function

Per #5809, you have to do some maneuvering just to print from a `#[start]` function, but if you don't know that, you get a very mysterious error at runtime:

``` rust
#[start]
fn main(_: int, _: **u8, _: *u8) -> int { 
  println("Hello!\n");
  return 0;
}
```

``` sh
$ ./hello

You've met with a terrible fate, haven't you?

fatal runtime error: runtime tls key not initialized
Abort trap: 6
```
