examples/download-file: Check return value for "file not found".
examples/download-file: Check return value for "file not found".

TimeDoctorLLC/dropbox-sdk-php,dropbox/dropbox-sdk-php,sayfulit/dropbox-sdk-php,itsazzad/dropbox-sdk-php,dropbox/dropbox-sdk-php,TimeDoctorLLC/dropbox-sdk-php,itsazzad/dropbox-sdk-php,sayfulit/dropbox-sdk-php,demisang/dropbox-sdk-php,demisang/dropbox-sdk-php