shell: Log failure in credentials.js load function
shell: Log failure in credentials.js load function

Print the ssh-add failure output to the console.log
when ssh-add fails. We need to parse and interpret both
stdout and stderr, but keep it legible.

Besides helping admins, this will help developers debug
the current ssh-add failure that sometimes happens during
testing.

Closes #3889
Reviewed-by: Peter <a316049dd34607c62d4611d63ab22a6454add223@redhat.com>

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