build.xml : add "failonerror=true" for exec steps

So that the ant build correctly reports that the
build failed if the exec failed.
Add xcshareddata for scheme management in Xcode

xcshareddata keeps track of the schemes available
for the xcworkspace, which is needed so that the
LauncherOSX scheme is available from the
LauncherOSX.xcworkspace and so that build.xml can
use the LauncherOSX.xcworkspace
build.xml : add derivedDataPath arg to xcodebuild

This allows xcodebuild to set the directory where
build products and other derived data will go