git-drover: Fix error printing.

Exceptions have no '.message' field.

Bug: 1135320
Change-Id: I30aba548895f1863c9cf65bf3e81a370405c32fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2451375
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

git_drover.py

@@ -461,7 +461,7 @@ def main():
                          options.parent_checkout, options.dry_run,
                          options.verbose)
   except Error as e:
-    print('Error:', e.message)
+    print('Error:', e)
     sys.exit(128)