Formatting CI prints diff on failure.

[1uc]

As a not very active contributor, it would be convenient if CI told me where I forgot to add that one " ". This can be faster than finding out which version of clang-format libsonata uses, installing it and then formatting the code. Naturally, in slightly more complex cases one simply needs to install clang-format. This mainly decreases the hurdle to submit tiny fixups of typos and the like.

If it doesn't interfere with other peoples workflow, I suggest printing the git diff.

[1uc]

Currently, the PR contains a commit which intentionally breaks formatting.

[mgeplf]

good call