rust.rc driver wrapping rustdoc does not propogate some errors

[pnkfelix]

% echo > tmp.rs && rustdoc tmp.rs --output-format html --output-dir foo ; echo $?
rust: ~"pandoc-out: "
rust: ~"pandoc-err: pandoc: foo/index.html: openFile: does not exist (No such file or directory)\n"
rust: task failed at 'pandoc failed', /Users/fklock/Dev/Mozilla/rust.git/src/librustdoc/markdown_writer.rs:121
rust: domain main @0x7fb163011a10 root task failed
101
% echo > tmp.rs && rust doc tmp.rs --output-format html --output-dir foo ; echo $?
0

[catamorphism]

I suspect this might be because fail doesn't set the exit code. Not a blocker IMO, anyway, but should be fixed.