Report better error message on update sandbox for gn builds

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=

Committed: https://crrev.com/d8807ee10dd5e1e534e46311383c51cc56999f42
Cr-Commit-Position: refs/heads/master@{#344600}

[IrfanGoogle, 2015-08-20 18:12:54]

isheriff@google.com changed reviewers:
+ miu@chromium.org

[IrfanGoogle, 2015-08-20 18:13:42]

On 2015/08/20 18:12:54, isheriff wrote:
> mailto:isheriff@google.com changed reviewers:
> + mailto:miu@chromium.org

Mostly a starter CL to get a hang of the workflow

[miu, 2015-08-20 21:19:17]

miu@chromium.org changed reviewers:
+ dpranke@chromium.org

[miu, 2015-08-20 21:19:18]

lgtm

Added an OWNER for approval.

[Dirk Pranke, 2015-08-20 21:32:08]

lgtm

[IrfanGoogle, 2015-08-20 21:33:42]

The CQ bit was checked by isheriff@google.com

[commit-bot: I haz the power, 2015-08-20 21:35:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1309433002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1309433002/1

[commit-bot: I haz the power, 2015-08-20 22:39:10]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-20 22:40:13]

Patchset 1 (id:??) landed as
https://crrev.com/d8807ee10dd5e1e534e46311383c51cc56999f42
Cr-Commit-Position: refs/heads/master@{#344600}