Auto merge of rust-lang#7008 - matthiaskrgr:ltchk, r=camsteffen

lintcheck: warn if we get a bad exit status while running clippy

Right now we won't notice if a crate fails to build.
Print a warning message to indicate that there is a problem of some sort.
I'll still have to do more investigation on why this actually happens.

I suspect that the problem is that `clippy fix` might run  --all-targets  but when we download the crate source from crates.io, some path deps (used for internal tests etc...) are not available (which is usually not a problem because the internal tests are not needed when using the crate as a lib..?)

changelog: none

Author: bors