[blinkpy] Log errors when trying to kill non-existent processes

More logging in hope of shedding some light on issue 929413
(content_shell outliving run_web_tests).

Bug: 929413
Change-Id: I47117a85a083158d13527a535dab312774220001
Reviewed-on: https://chromium-review.googlesource.com/c/1474630Reviewed-by: Quinten Yearsley <qyearsley@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632388}

third_party/blink/tools/blinkpy/common/system/executive.py

@@ -134,7 +134,7 @@ class Executive(object):
 
             command = ['taskkill.exe', '/f', '/t', '/pid', pid]
             # taskkill will exit 128 if the process is not found. We should log.
-            self.run_command(command, error_handler=self.ignore_error)
+            self.run_command(command, error_handler=self.log_error)
             return
 
         try:
@@ -142,7 +142,7 @@ class Executive(object):
             os.waitpid(pid, os.WNOHANG)
         except OSError as error:
             if error.errno == errno.ESRCH:
-                # The process does not exist.
+                _log.debug("PID %s does not exist.", pid)
                 return
             if error.errno == errno.ECHILD:
                 # Can't wait on a non-child process, but the kill worked.
@@ -265,6 +265,10 @@ class Executive(object):
     def ignore_error(error):
         pass
 
+    @staticmethod
+    def log_error(error):
+        _log.debug(error)
+
     def _compute_stdin(self, user_input):
         """Returns (stdin, string_to_communicate)"""
         # FIXME: We should be returning /dev/null for stdin