core: improve error msg text in storage connection

Improve error message text for case when storage connection doesn't exist:
Remove "internal error" because it doesn't have to be an internal error,
it could be that the user entered not existing nfs path, and having
"internal error" in message in UI is misleading the user.
Also make "Connection" to "conection" because it's in the middle of a
sentence and it shouldn't be a capital letter

Change-Id: Icec819a8cf91e04051ce77244f79bb85db5f8972
Signed-off-by: Alissa Bonas <abonas@redhat.com>