Is there any alternative that will auto format the code, instead of just throwing an error?
If the diff is full of additional formatting/stylistic changes, it can make it a lot harder to cut through the noise and get down to the heart of the change.
I'm not saying you should never clean up old code/files, however when I see that needs to be done, I try to do it in a separate commit that I make after the "real" change. The log message for that change is something like "Formatting", or "Cleanup, no functional changes".
However, because the entire code-base was cleaned up at once we now have no additional noise from Uncrustify being run on our commits. The only changes to style it will ever suggest are with your current changes you are attempting to commit since everything in the repo right now is "clean".