Server/Admin: fatal errors should go to stderr

If an error occurs, that leads to an termination of the process,
this error should be printed to stderr.
Merge pull request #183 from AlexanderS/errexit-stderr

Server/Admin: fatal errors should go to stderr
Packages/Apt: Essential could be "no"

The "Essential" field in the package control fields could be "yes" or "no".
Only yes sould define the package as essential. The value "no" sould be
handled same as not having the field at all.
doc: Fix typo

Signed-off-by: Sol Jerome <sol.jerome@gmail.com>