auto merge of #11644 : huonw/rust/less-fatality, r=cmr

This means that compilation continues for longer, and so we can see more
errors per compile. This is mildly more user-friendly because it stops
users having to run rustc n times to see n macro errors: just run it
once to see all of them.

diff --cc src/libsyntax/ext/expand.rs
Simple merge

diff --cc src/libsyntax/ext/format.rs
Simple merge