record middleware error on the dependency, rather than throwing

Step 1 for https://github.com/normalize/nlz/issues/15

Other steps would be to
* display all the recorded errors
* abort the build, as to not overwrite with invalid files

I would suggest to do something like makes --keep-going, as to record as many errors as possible instead of aborting at the first one.
I’m not really sure where the error is exactly. Or maybe you rethrow it somewhere else too that I haven’t figured out yet.