Catch symlink creation failures and report a better error

Some users have noticed that repo doesn't work on VFAT, as we
require a POSIX filesystem with POSIX symlink support.  Catching the
OSError during our symlink creation and raising a GitError with a
more descriptive message will help users to troubleshoot and fix
their own installation problems.

Signed-off-by: Shawn O. Pearce <sop@google.com>
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