Print formatting error

Dockerfile

@@ -14,6 +14,7 @@ RUN apt-get install -y \
   git \
   python3 \
   python3-pip \
+  clang-format \
   wget --no-install-recommends && \
   apt-get clean
 RUN python3 -m pip install bdflib

source/Makefile

@@ -535,6 +535,7 @@ check-style:
 		var=`clang-format "$$src" | diff "$$src" - | wc -l` ; \
 		if [ $$var -ne 0 ] ; then \
 			echo "$$src does not respect the coding style (diff: $$var lines)" ; \
+			clang-format "$$src" | diff "$$src" - \
 			exit 1 ; \
 		fi ; \
 	done