core: fix application messages

Some error messages are enclosed in double quotation marks that are
redundant and one message contains a typo

Change-Id: I898c232be9ad082ec262f44e2a958763e405bac0
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1131484
Signed-off-by: Eli Mesika <emesika@redhat.com>