msvs: ensure that failing actions make the build fail with multiple actions

The old way of joining multiple commands would take care of failing correctly
when there are multiple actions associated with a single file. However, for
multiple actions associated with different files (but within the same target),
Visual Studio will still create just one .bat file for them all, so we must
make sure that each command errors out correctly when it fails.

Strictly speaking, the %errorlevel% check is unnecessary for the last command
in a target (because the .bat file created by VS contains it already); but it
doesn't hurt either, and makes the logic much simpler.

Patch from Stefan Haller <stk.haller@googlemail.com>.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/335273006

git-svn-id: http://gyp.googlecode.com/svn/trunk@1945 78cadc50-ecff-11dd-a971-7dbc132099af