sshpeer: add a configurable hint for the ssh error message
Adding a possibility to configure error hint to be shown in the case of problems with SSH. Example of such hint can be "Please see http://company/internalwiki/ssh.html".
Test Plan:
- Ran hg pull with broken link and verified the output has no hint by default:
```
pulling from ssh://brokenrepository.com//repo
remote: ssh: Could not resolve hostname brokenrepository.com: Name or service not known
abort: no suitable response from remote hg!
```
- Run hg pull --config ui.ssherrorhint="Please see http://company/internalwiki/ssh.html":
```
pulling from ssh://brokenrepository.com//repo
remote: ssh: Could not resolve hostname brokenrepository.com: Name or service not known
abort: no suitable response from remote hg!
(Please see http://company/internalwiki/ssh.html)
```
Differential Revision: https://phab.mercurial-scm.org/D1431
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