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

[jsanders]

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:

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

$ ./hello

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

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

[alexcrichton]

This is working today, closing. (there's a test somewhere in the test suite for this)

[jsanders]

Thanks! Sorry I hadn't kept my eye on it.