Catch and log any exceptions while closing the transport

Previously these would show up as big tracebacks in the logs which
was ugly, considering all sorts of things can go wrong when suddenly
closing the transport (such as when a celery worker terminates)

Fixes elastic#823

elasticapm/transport/base.py

@@ -70,7 +70,7 @@ def __init__(
         queue_chill_count=500,
         queue_chill_time=1.0,
         processors=None,
-        **kwargs
+        **kwargs,
     ):
         """
         Create a new Transport instance
@@ -132,7 +132,13 @@ def _process_queue(self):
 
             if event_type == "close":
                 if buffer_written:
-                    self._flush(buffer)
+                    try:
+                        self._flush(buffer)
+                    except Exception as exc:
+                        logger.error(
+                            "Exception occurred while flushing the buffer "
+                            "before closing the transport connection: {0}".format(exc)
+                        )
                 self._flushed.set()
                 return  # time to go home!
 
@@ -257,7 +263,7 @@ def close(self):
         self._closed = True
         self.queue("close", None)
         if not self._flushed.wait(timeout=self._max_flush_time):
-            raise ValueError("close timed out")
+            logger.error("Closing the transport connection timed out.")
 
     stop_thread = close