Use set -x and proper exit code

.travis/pep8.sh

 #!/bin/sh
 
+set -x
+
 # Run pep8 on all .py files in all subfolders
 # We must ignore E402 module level import not at top of file
 # because of use case sys.path.append('..'); import <module>
@@ -19,7 +21,8 @@ num_errors_after=`cat $tmpafter | wc -l`
 echo $num_errors_after
 
 if [ $num_errors_after -gt $num_errors_before ]; then
-	echo "New PEP8 errors were introduced: $(diff -u $tmpbefore $tmpafter)"
-	return 1
+	echo "New PEP8 errors were introduced:"
+	diff -u $tmpbefore $tmpafter
+	exit 1
 fi