SSHDriver: Fix Timeout handling on start_own_master()

Without this change an Exception during `_start_own_master_once()`
could get lost if the timeout happens at the wrong point in time.

With this change we re-raise the last exception that happened during
`_start_own_master_once()`.

Signed-off-by: Chris Fiege <chris@tinyhost.de>
(cherry picked from commit 9db6b79)

Author: SmithChart

labgrid/driver/sshdriver.py

@@ -97,13 +97,11 @@ def _start_own_master(self):
         # connection refused during target startup
         connect_timeout = round(timeout.remaining)
         while True:
-            if connect_timeout == 0:
-                raise Exception("Timeout while waiting for ssh connection")
             try:
                 return self._start_own_master_once(connect_timeout)
             except ExecutionError as e:
                 if timeout.expired:
-                    raise e
+                    raise Exception("Timeout while waiting for ssh connection") from e
                 time.sleep(0.5)
                 connect_timeout = round(timeout.remaining)