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

mercurial/configitems.py

@@ -964,6 +964,9 @@ coreconfigitem('ui', 'slash',
 coreconfigitem('ui', 'ssh',
     default='ssh',
 )
+coreconfigitem('ui', 'ssherrorhint',
+    default=None,
+)
 coreconfigitem('ui', 'statuscopies',
     default=False,
 )

mercurial/help/config.txt

@@ -2188,6 +2188,10 @@ User interface controls.
 ``ssh``
     Command to use for SSH connections. (default: ``ssh``)
 
+``ssherrorhint``
+    A hint shown to the user in the case of SSH error (e.g.
+    ``Please see http://company/internalwiki/ssh.html``)
+
 ``strict``
     Require exact command names, instead of allowing unambiguous
     abbreviations. (default: False)

mercurial/sshpeer.py

@@ -204,8 +204,9 @@ class sshpeer(wireproto.wirepeer):
         self._pipeo = doublepipe(self.ui, self._pipeo, self._pipee)
 
         def badresponse():
-            self._abort(error.RepoError(_('no suitable response from '
-                                          'remote hg')))
+            msg = _("no suitable response from remote hg")
+            hint = self.ui.config("ui", "ssherrorhint")
+            self._abort(error.RepoError(msg, hint=hint))
 
         try:
             # skip any noise generated by remote shell

tests/test-ssh.t

@@ -578,3 +578,20 @@ abort during pull is properly reported as such
   remote: abort: this is an exercise
   abort: pull failed on remote
   [255]
+
+abort with no error hint when there is a ssh problem when pulling
+
+  $ hg pull ssh://brokenrepository
+  pulling from ssh://brokenrepository/
+  remote: ssh: Could not resolve hostname brokenrepository: Name or service not known
+  abort: no suitable response from remote hg!
+  [255]
+
+abort with configured error hint when there is a ssh problem when pulling
+
+  $ hg pull ssh://brokenrepository --config ui.ssherrorhint="Please see http://company/internalwiki/ssh.html"
+  pulling from ssh://brokenrepository/
+  remote: ssh: Could not resolve hostname brokenrepository: Name or service not known
+  abort: no suitable response from remote hg!
+  (Please see http://company/internalwiki/ssh.html)
+  [255]