Description of problem:

Some vault error message can be improved to be more helpful and descriptive.

How reproducible:
Always.


Steps to Reproduce:

1. Try to add a new vault owner as a regular user without vault admin
privileges:
$ ipa vault-add-owner vault_name --users user_name
...
    Failed owners: owner user: user_name: Insufficient access: Insufficient
'write' privilege to the 'owner' attribute of entry
...

2. Try to display information about a vault in a container other than your own
private container. For example, run ipa vault-show on a shared vault without
adding --shared:
$ ipa vault-show shared_vault
ipa: ERROR: shared_vault: vault not found


Actual results:
The current error messages don't clearly explain what is wrong and how to fix
the problem.

Expected results:

In case of 1., the error message could clearly say that the user doesn't have
sufficient privileges for the command.

In case of 2., the error message could say that by default, users can only use
the command on vaults in their private container. If they want to target a
vault in another container, they must add additional options (--shared, --user,
or --service).


Additional info: