Guard against configurations without 'xcode_settings'
Guard against configurations without 'xcode_settings'

e.g. test/generator-output/src/prog1.gyp

TEST=Run
    ./gyptest.py -f xcode-ninja test/generator-output/gyptest-top-all.py

Expected to fail with
    OSError: [Errno 13] Permission denied: 'prog1.ninja.xcworkspace'
(instead of KeyError: 'xcode_settings')

R=sdefresne@chromium.org, justincohen@chromium.org

Review URL: https://codereview.chromium.org/428913002
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google