process: Improve launch_sync() exception due to process failing

After this commit, in some situations ssh related errors are printed
directly in the exception to quickly find cause of the issue.

Example:
    FAIL: ping.py (5.0 sec) Error: rm-remote-dir(pid=25913): launch_sync(): local ssh process exited with status 255 (ssh: connect to host 10.42.42.110 port 22: No route to host) [trial↪4g:srsue-rftype@zmq+srsenb-rftype@zmq+mod-enb-nprb@6↪ping.py:9↪ping.py↪srsepc_10.42.42.118↪host-jenkins@10.42.42.110↪rm-remote-dir(pid=25913)]

Change-Id: Ia16c7dec96f70d761600ad6a50d9df8382d9c2c8
diff --git a/src/osmo_gsm_tester/core/process.py b/src/osmo_gsm_tester/core/process.py
index 36ed057..33ae69a 100644
--- a/src/osmo_gsm_tester/core/process.py
+++ b/src/osmo_gsm_tester/core/process.py
@@ -196,7 +196,7 @@
             self.terminate()
             raise e
         if raise_nonsuccess and self.result != 0:
-            raise log.Error('Exited in error %d' % self.result)
+            raise self.RunError('launch_sync()')
         return self.result
 
     def respawn(self):
@@ -357,6 +357,11 @@
             self.process_obj.stdin.write(cmd.encode("utf-8"))
             self.process_obj.stdin.flush()
 
+    def RunError(self, msg_prefix):
+        'Get a log.Error filled in with Result information. Use when program is terminated and result !=0'
+        msg = '%s: local process exited with status %d' % (msg_prefix, self.result)
+        return log.Error(msg)
+
 class RemoteProcess(Process):
 
     def __init__(self, name, run_dir, remote_user, remote_host, remote_cwd, popen_args, remote_env={}, **popen_kwargs):
@@ -382,6 +387,17 @@
                                          ' '.join(self.popen_args))]
         self.dbg(self.popen_args, dir=self.run_dir, conf=self.popen_kwargs, remote_env=self.remote_env)
 
+    def RunError(self, msg_prefix):
+        'Overwrite Process method with ssh extra information'
+        # man ssh states it returns 255 if an ssh error occurred:
+        msg = msg_prefix + ': '
+        if self.result == 255:
+            tail = ' (' + (self.get_stderr_tail(tail=1, prefix='') or '').rstrip() + ')'
+            msg += 'local ssh process exited with status %d%s' % (self.result, tail if 'ssh' in tail else '')
+        else:
+            msg += 'remote process exited with status %d' % (self.result)
+        return log.Error(msg)
+
 class NetNSProcess(Process):
     NETNS_EXEC_BIN = 'osmo-gsm-tester_netns_exec.sh'
     def __init__(self, name, run_dir, netns, popen_args, **popen_kwargs):