Fix bug in postinst.

git-svn-id: http://svn.code.sf.net/p/openmediavault/code/trunk@1041 3c3d7b80-5828-415e-9f61-280b98178fff
* Redirect error messages
* Improve task handling and fix JSON file issue.

git-svn-id: http://svn.code.sf.net/p/openmediavault/code/trunk@1042 3c3d7b80-5828-415e-9f61-280b98178fff