Exit status on `ssh-add` failure

Hi,

I'm developing a library for building custom SSH agents and noticed that 
OpenSSH's `ssh-add` doesn't return non-zero exit code on some failures:

$ SSH_AUTH_SOCK=/tmp/test.sock ssh-add client
Identity added: client (test-client)
Certificate client-cert.pub (client) add failed: communication with 
agent failed
$ echo $?
0

The certificate has not been added due to agent protocol error but 
ssh-add still returned exit code 0.

The EXIT STATUS section of the manpage for ssh-add states: "Exit  status 
  is  0  on success, 1 if the specified command fails, and 2 if ssh-add 
is unable to contact the authentication agent." but it doesn't mention 
what happens when the command partially succeeds (identity has been 
added but the certificate not).

Is that the intended behavior of ssh-add?

Thank you for your time and help!

Kind regards,
Wiktor

For reference, the link to my user's report:
https://github.com/wiktor-k/ssh-agent-lib/pull/101#issuecomment-4259465799