User plugin: improve error related to non existing idp
    
    The user and stageuser commands return the following error
    when the user is created/updated with a non existing idp:
    $ ipa user-add testuser --first test --last user --idp dummy
    ipa: ERROR: no such entry
    
    The error is not descriptive enough and has been modified to
    display instead:
    $ ipa user-add testuser --first test --last user --idp dummy
    ipa: ERROR: External IdP configuration dummy not found
    
    Fixes: https://pagure.io/freeipa/issue/9416
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>