Feature: only display output from Log in case of failure

I am using `Log` automatically during Neomake's tests to log everything (https://github.com/neomake/neomake/blob/06cd1ac39f5cbd403e92732bce7e8ae66ded1fcd/autoload/neomake/utils.vim#L26-L31), but that tends to get noisy.

What do you think about a way to only display the output from `Log` in case the test failed?

This would be similar to what pytest does by default with the captured stdout/stderr: it only gets displayed (in non-verbose mode) in case the test failed.