The documentation for gn builds specifies using a different
output directory than gyp builds, but the update sandbox script
reports error message assuming only gyp builds.
Update the error message to prod user into specifying BUILDTYPE
to find the sandbox in the right place
BUG=
Review URL: https://codereview.chromium.org/1309433002
Cr-Original-Commit-Position: refs/heads/master@{#344600}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d8807ee10dd5e1e534e46311383c51cc56999f42