Add context to "file must be sorted" error message in ios/PRESUBMIT.py

BUG=none

Review URL: https://codereview.chromium.org/168643003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251664 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251664