Disabled a vim trace.

Leo build: 20140824181150
https://bugs.launchpad.net/leo-editor/+bug/1260415

Added more informative error messages in at.writeAllAtFileNodesHelper.
This is the direct response to the bug report.  A typical message::

    Internal error writing one or more external files.
    Please report this error to:
    https://groups.google.com/forum/#!forum/leo-editor
    All changes will be lost unless you
    can save each changed file.

Also added a much better fix: catch all exceptions in at.writeAll.
A typical message::

    Internal error writing: @<file> node
    Please report this error to:
    https://groups.google.com/forum/#!forum/leo-editor
    Warning: changes to this file will be lost
    unless you can save the file successfully.

Leo build: 20140824182351