Rustdoc --test does not fail if tests fail

[alexcrichton]

$ cat foo.rs
#![crate_type = "lib"]
///
/// ```
/// fail!();
/// ```
pub fn foo() {}
$ rustdoc --test foo.rs

running 1 test
test foo_0 ... FAILED

failures:

---- foo_0 stdout ----
        task 'foo_0' failed at 'test executable failed:
        Some(task '<main>' failed at 'explicit failure', <anon>:5
        )', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/librustdoc/test.rs:204



failures:
    foo_0

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured

task '<unnamed>' failed at 'Some tests failed', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libt
est/lib.rs:242
$ echo $?
0