Fix screenshot error when no screenshot is requested.

BUG=chromium:237142,chromium:239890
TEST=Verfied spurious error message is fixed.

Change-Id: I77129990f85e324e67066a55e2b0d4ecbd7d58d2
Reviewed-on: https://gerrit.chromium.org/gerrit/50980
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Commit-Queue: WeiNan-Peter, Wen <wnwen@chromium.org>
Tested-by: WeiNan-Peter, Wen <wnwen@chromium.org>
4 files changed
tree: 6a71658fe5ef4ba49266257e3211ff33063b096d
  1. mtedit/
  2. mtlib/
  3. __init__.py
  4. common.mk
  5. Makefile