List modified bmv2 artifacts in fabric.p4 verify jobs

To facilitate debugging from the Jenkins console output

Change-Id: I0d3e043b831118f2de9001709adc2a975000c260
diff --git a/jjb/shell/fabric-p4/fabric-p4-build-bmv2.sh b/jjb/shell/fabric-p4/fabric-p4-build-bmv2.sh
index 53c5a80..caead84 100644
--- a/jjb/shell/fabric-p4/fabric-p4-build-bmv2.sh
+++ b/jjb/shell/fabric-p4/fabric-p4-build-bmv2.sh
@@ -28,8 +28,11 @@
 make all
 
 echo "Step 2 - Checking bmv2 artifacts"
-if [ -n "$(git status --porcelain)" ]; then
-    echo "The uploaded P4 build artifacts do not correspond to the expected ones. Please run the Makefile locally before pushing a a new change"
+modified=$(git status --porcelain)
+if [ -n "$modified" ]; then
+    echo "The following P4 build artifacts do not correspond to the expected ones,"
+    echo "please run the Makefile locally before pushing a new change:"
+    echo "$modified"
     exit 1
 fi