feat(libstore): print the first line of stdout of SSH in case of failure

In case of failure to connect as can be seen in
https://buildbot.lix.systems/#/builders/39/builds/1386/steps/1/logs/stdio

It is difficult to understand what happened, if we enabled the talkative
verbose level, we could learn about the first line SSH sent us.

In practice, this is not workable, we can just make it warn all the
time.

Change-Id: Iaaf56894060a58f2dfc78254bb60b1c43482f9bb
Signed-off-by: Raito Bezarius <raito@lix.systems>

src/libstore/ssh.cc

@@ -110,7 +110,7 @@ std::unique_ptr<SSHMaster::Connection> SSHMaster::startCommand(const std::string
         } catch (EndOfFile & e) { }
 
         if (reply != "started") {
-            printTalkative("SSH stdout first line: %s", reply);
+            warn("SSH to '%s' failed, stdout first line: '%s'", host, reply);
             throw Error("failed to start SSH connection to '%s'", host);
         }
     }