Error out with a useful message if a mapped file in unversal doesn't exist.

From https://github.com/sbt/sbt-native-packager/issues/23
Implemented chown file permissions and user/group creation
Ticket #62: File Permissions - user isn't set?
Verify mapped files exist in settings for all configs.
Merge pull request #86 from kardapoltsev/symlink-fix

error creating symlink when file already exists fixed
Merge pull request #85 from kardapoltsev/master

Sysvinit script for debian package
Merge branch 'master' of github.com:sbt/sbt-native-packager into sbt13
Merge pull request #84 from muuki88/master

Implemented chown file permissions and user/group creation
Merge pull request #83 from gmjabs/master

Informative failure for missing files mapped in universal
restart option added to sysvinit script, bug with wrong name for sysvinit script fixed
Merge branch 'master' into sbt13
small bug fix
Merge branch 'master' into sbt13
sysvinit template updated
Merge branch 'master' into sbt13
sh changed to bash
Merge branch 'master' into sbt13
Merge pull request #89 from kardapoltsev/master

Restart option added to sysvinit script
generating etc default, different postinst script for sysvinit
Merge branch 'wip/etc-default' into sbt13
etc default added to conffiles