Bug 1622637 - Fix exception reraising in mozrunner r=gbrown

Differential Revision: https://phabricator.services.mozilla.com/D66895

--HG--
extra : moz-landing-system : lando

testing/mozbase/mozrunner/mozrunner/base/runner.py

@@ -131,9 +131,11 @@ class BaseRunner(object):
                 process.run(self.timeout, self.output_timeout)
 
                 self.process_handler = process
-            except Exception:
-                _, value, tb = sys.exc_info()
-                reraise(RunnerNotStartedError, "Failed to start the process: %s" % value, tb)
+            except Exception as e:
+                reraise(RunnerNotStartedError,
+                        RunnerNotStartedError(
+                            "Failed to start the process: {}".format(e)),
+                        sys.exc_info()[2])
 
         self.crashed = 0
         return self.process_handler.pid

testing/mozbase/mozrunner/tests/test_start.py

@@ -5,6 +5,10 @@ from __future__ import absolute_import
 from time import sleep
 
 import mozunit
+from mock import patch
+from pytest import raises
+
+from mozrunner import RunnerNotStartedError
 
 
 def test_start_process(runner):
@@ -48,5 +52,13 @@ def test_start_with_outputTimeout(runner):
     assert not runner.is_running()
 
 
+def test_fail_to_start(runner):
+    with patch('mozprocess.ProcessHandler.__init__') as ph_mock:
+        ph_mock.side_effect = Exception('Boom!')
+        with raises(RunnerNotStartedError):
+            runner.start(outputTimeout=0.1)
+            sleep(1)
+
+
 if __name__ == '__main__':
     mozunit.main()