dir/server/tree: fix bug handling erroneous argument to Watch
dir/server/tree: fix bug handling erroneous argument to Watch

The API requires us to report an invalid order parameter on the
channel, not in the call. This was broken in the recent flurry of
changes.

Should fix the build.

Change-Id: Id6c2d4ec3ed5ae719115bbcafbd64ded84b3855b
Reviewed-on: https://upspin-review.googlesource.com/14420
Reviewed-by: David Presotto <d244755566b06d4c956c05eb7fe29c9fb345f41e@gmail.com>

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