error message for start function confusing

[metajack]

If you had an old start function like:

#[start]
fn start(argc: int, argv: **u8, crate_map: *u8) -> int {
 // ...
}

You this message:
error: start function expects type: extern "Rust" fn(int, **u8) -> int: incorrect number of function parameters

However, if actually change the signature to extern "Rust" fn start(argc: int, argv: **u8) -> int` then you get the this message:

error: start function expects type: extern "Rust" fn(int, **u8) -> int: expected impure fn but found extern fn

It took me a bit to figure out that I am not supposed to add extern "Rust" myself.

cc @brson

[huonw]

Triage: now fixed, closed when #11966 lands.