Report better error message on update sandbox for gn builds

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-Commit-Position: refs/heads/master@{#344600}
1 file changed