Count "failed to run custom build command" as error?

When debugging a project, I opened bacon alongside my editor as usual. bacon said:

```
[ripasso] [check] [1 warning]
```

but then I found out that one of the dependencies failed to `build.rs`:

```
error: failed to run custom build command for `libgpg-error-sys v0.6.2`

Caused by:
process didn't exit successfully: `/home/i/works/debcargo-conf/build/ripasso/target/debug/build/libgpg-error-sys-9b3670d237bb0344/build-script-build` (exit status: 1)
--- stdout
(.. omitted)
--- stderr
Error: PkgConfig(
pkg-config exited with status code 1
> PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=1 pkg-config --libs --cflags gpg-error 'gpg-error >= 1.12'

The system library `gpg-error` required by crate `libgpg-error-sys` was not found.
(.. omitted)
)
```

IMHO it would be helpful if such situations could be detected and counted as an error.