Fix error in error message concatenation in jumbo script.

Best to not try to add a string to a set without converting the set to
a string first.

R=dpranke@chromium.org

Bug: 713137
Change-Id: Id4c632b1f944fcafdd046983b1ddd4678c44928f
Reviewed-on: https://chromium-review.googlesource.com/564939Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: bratell at Opera <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#486706}

build/config/merge_for_jumbo.py

@@ -83,7 +83,8 @@ def main():
   header_files = set([x for x in all_inputs if x.endswith(".h")])
   assert set(args.outputs) == written_output_set, "Did not fill all outputs"
   files_not_included = set(all_inputs) - written_input_set - header_files
-  assert not files_not_included, "Did not include files: " + files_not_included
+  assert not files_not_included, ("Jumbo build left out files: %s" %
+                                  files_not_included)
   if args.verbose:
     print("Generated %s (%d files) based on %s" % (
       str(args.outputs), len(written_input_set), args.file_list))