bluelet: catch ECONNRESET

When ncmpcpp quits after an error it causes a "connection reset by peer"
exception, also known as ECONNRESET (104) in errno terms. In Python
2 this is mapped to a `socket.error` and in Python 3 this is
  `ConnectionResetError` which is thankfully a subclass of the
  `socket.error` exception class.

beets/util/bluelet.py

@@ -346,6 +346,10 @@ def run(root_coro):
                             exc.args[0] == errno.EPIPE:
                         # Broken pipe. Remote host disconnected.
                         pass
+                    elif isinstance(exc.args, tuple) and \
+                            exc.args[0] == errno.ECONNRESET:
+                        # Connection was reset by peer.
+                        pass
                     else:
                         traceback.print_exc()
                     # Abort the coroutine.