snap ack doesn't indicate which assertions are good or bad

[Ryan Harper]

1. % lsb_release -rd
Description: Ubuntu 16.04.1 LTS
Release: 16.04

2. % apt-cache policy snapd
snapd:
  Installed: 2.16
  Candidate: 2.16
  Version table:
 *** 2.16 100
        100 /var/lib/dpkg/status
     2.15.2ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu xenial-updates/main amd64 Packages
     2.0.2 500
        500 http://us.archive.ubuntu.com/ubuntu xenial/main amd64 Packages

3. snap ack <file with list of assertions, one with an error>

This should ack the assertions that are valid, and indicate which one of the
assertions was invalid; it's also unclear if any of the assertions that were
valid were imported or not since there's not snap known --list output.

One can manually snap known <assertion type> but it's not very friendly

4. snap ack exits with a failure message but without enough information to
understand which assertion failed if the file includes multiple assertions
of the same type.

% snap download hello
% use $EDITOR to modify hello_20.snap.assertions somewhere in the middle (change a byte in the base64 encoded values)
% sudo snap ack assertions
error: cannot assert: assert failed: cannot add some assertions to the system database:
 - cannot decode signature: illegal base64 data at input byte 718