Keepassxc: Gracefully handle known error messages in SSH-Agent

[droidmonkey]

Expected Behavior

A helpful error message should be presented when known errors from pageant, gnome-keyring, or ssh-agent are returned. The affected keys / entries should also be easily opened from the error message itself.

Current Behavior

An often cryptic error message is presented to the user (see #2762 for example)

Possible Solution

Perform limited parsing of error strings returned by key storage applications to help assist the user in correcting the issue.

[hifi]

AFAIK the protocol has no error strings and the presented error was in the debug output of the agent itself in #2762. That's why it's hard to know why something failed, it just does.

https://tools.ietf.org/html/draft-miller-ssh-agent-02#section-4.1

EDIT: Also the "protocol error" comes when the agent does not reply with a fail message (where we have a helpful note about constraints) but just disconnects immediately which is very broken behavior in older gnome-keyring releases.

[hifi]

Closing as we can't do much without any error messages from the protocol.