xcode generator: warn if QMAKE_INFO_PLIST is not valid

Warn the user if QMAKE_INFO_PLIST is set, but file not found.
An iOS application will not run or deploy without an
Info.plist present, and the error message given by
xcodebuild is not very informative.

Change-Id: I54f0e06de320a43c9f3261fe88761c41e3ccd022
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>