Whenever Cargo tries to build a project while there is another build in
progress, Cargo will wait for the other build to finish, and print
messages like this:

```
Blocking waiting for file lock on package cache
Blocking waiting for file lock on package cache
Blocking waiting for file lock on build directory
```

Those messages were not ignored, and were therefore included in the
quickfix, making it look like there were some issues even if the build
succeeded.

This commit configures `&errorformat` to ignore those messages and avoid
including them in the quickfix.

Closes rust-lang#499 .