[MacPorts] #54670: Improve error report for MacVim mvim script for a bad link

[MacPorts]

#54670: Improve error report for MacVim mvim script for a bad link
---------------------+-----------------
 Reporter:  decibel  |      Owner:
     Type:  defect   |     Status:  new
 Priority:  Normal   |  Milestone:
Component:  ports    |    Version:
 Keywords:           |       Port:
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 If a symlink to the mvim script has the wrong permissions (sadly easy to
 do on OS X), it fails with some rather confusing error messages. The
 change below fixes this:

     while [ -L "$self_path" ]; do
         next_path=`readlink $self_path` || { echo "error $? reading link
 $self_path (are permissions on the link correct?)" 2>&1; exit 2; }
         cd "`dirname $next_path`"
         self_path=`basename $next_path`
     done

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Ticket URL: <https://trac.macports.org/ticket/54670>
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