Rework on child process management.

When receiving the SIGTERM signal, swallow it and terminate latest created child process
Catch ChildProcessError (occurs when child process is terminated by the current python script on mac os x)

bazarr.py

@@ -2,6 +2,7 @@
 
 import os
 import platform
+import signal
 import subprocess
 import sys
 import time
@@ -37,11 +38,18 @@ def end_child_process(ep):
     except:
         pass
 
+def terminate_child_process(ep):
+    try:
+        ep.terminate()
+    except:
+        pass
+
 
 def start_bazarr():
     script = [sys.executable, "-u", os.path.normcase(os.path.join(dir_name, 'bazarr', 'main.py'))] + sys.argv[1:]
     ep = subprocess.Popen(script, stdout=None, stderr=None, stdin=subprocess.DEVNULL)
     atexit.register(end_child_process, ep=ep)
+    signal.signal(signal.SIGTERM, lambda signal_no, frame: terminate_child_process(ep))
 
 
 def check_status():
@@ -92,6 +100,6 @@ if __name__ == '__main__':
             else:
                 os.wait()
                 time.sleep(1)
-        except (KeyboardInterrupt, SystemExit):
+        except (KeyboardInterrupt, SystemExit, ChildProcessError):
             print('Bazarr exited.')
             sys.exit(0)