ENH: add messages for ssh-agent-helper exit state to make the result clear

[ZLLentz]

Reviewers: I mostly want quick opinions about the word choice and the terminal output, you shouldn't spend a lot of time out of your day on this one. I think this is the normal/simplest way to check a return code in a bash script.

Description

If the ssh-add step in ssh-agent-helper runs, let the user know how it goes.
The script remains silent if it didn't need to do anything.

Motivation and Context

This came up when @ljansen7 was deploying IOCs a week or so ago and the script output was unclear: did it work? Did it not work? Do I have to do it again?

Jira at https://jira.slac.stanford.edu/browse/ECS-6095

How Has This Been Tested?

I ran the script a few times with different conditions:

Correct password:

zlentz@pslogin04:~$ source ~/github/engineering_tools/scripts/ssh-agent-helper
Starting ssh agent
Running ssh-add, may prompt for ssh key password
Enter passphrase for /cds/home/z/zlentz/.ssh/id_rsa:
ssh-agent-helper complete: key successfully added

Wrong password a bunch, ctrl+c and skips my message

zlentz@pslogin02:~$ source ~/github/engineering_tools/scripts/ssh-agent-helper
Starting ssh agent
Running ssh-add, may prompt for ssh key password
Enter passphrase for /cds/home/z/zlentz/.ssh/id_rsa:
Bad passphrase, try again for /cds/home/z/zlentz/.ssh/id_rsa:
Bad passphrase, try again for /cds/home/z/zlentz/.ssh/id_rsa:
Bad passphrase, try again for /cds/home/z/zlentz/.ssh/id_rsa:
Bad passphrase, try again for /cds/home/z/zlentz/.ssh/id_rsa:

Empty password, but actually my key is encrypted still and the return code catches it

zlentz@pslogin02:~$ source ~/github/engineering_tools/scripts/ssh-agent-helper
Running ssh-add, may prompt for ssh key password
Enter passphrase for /cds/home/z/zlentz/.ssh/id_rsa:
ssh-agent-helper error: at least one key failed to be added

Where Has This Been Documented?

Here only, later in release notes

[tangkong]

Seems like a net positive to me 👍