make prematurely exited processes fail the test

Change-Id: I54394c40718b44378df597e32003742059052869
diff --git a/src/osmo_gsm_tester/suite.py b/src/osmo_gsm_tester/suite.py
index 0b8927f..2d6c67b 100644
--- a/src/osmo_gsm_tester/suite.py
+++ b/src/osmo_gsm_tester/suite.py
@@ -250,10 +250,14 @@
         self.wait(lambda: False, timeout=seconds)
 
     def poll(self):
+        ofono_client.poll()
         if self._processes:
             for process in self._processes:
                 process.poll()
-        ofono_client.poll()
+                if not process.is_running():
+                    process.log_stdout_tail()
+                    process.log_stderr_tail()
+                    process.raise_exn('Process ended prematurely')
 
     def prompt(self, *msgs, **msg_details):
         'ask for user interaction. Do not use in tests that should run automatically!'