diffchecks.sh: Make success clear and exit with failure on error

scripts/diffcheck.sh

@@ -9,6 +9,8 @@ if git checkout HEAD^ && scripts/buildtable.pl >/tmp/table.mediawiki 2>/dev/null
         echo "$newdiff"
         exit 1
     fi
+    echo "README table matches expected table from BIP files"
 else
     echo 'Cannot build previous commit table for comparison'
+    exit 1
 fi