enter_chroot: add more helpful error message when using CHROME_ROOT

No functional changes, just clarifies error message for users when
things go wrong.

BUG=None
TEST=`cros_sdk` works

Change-Id: I20bb736e757da556c1956e828e7198e71c8d026a
Reviewed-on: https://chromium-review.googlesource.com/178331
Reviewed-by: Matt Tennant <mtennant@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
1 file changed