Catch to prevent push-receiver crash on disconnect

[tsightler]

This patch addresses an unhandled promise rejection that occurs in push-receiver 4.3.0. This version of push-receiver introduced a new whenReady promise that consumers can use to know when the initial connection is ready. However, it seems to have a bug when the socket connection fails as the code rejects the promise but has no catch for this rejection, thus creating an unhandled promise which leads to a process crash on modern NodeJS >15 which exit on unhandled promises by default.

It's possible to attach a catch handler outside of the library during first use, but the issue is that a new promise is created on every connection attempt, so, while that will catch the initial failure, subsequent retries are new promises without a catch.

This patch takes a different approach, instead of attaching the catch handler during creating, it instead listens for "ON_DISCONNECT" event, which happens just before the promise rejection, and immediately attaches a catch handler to the promise to prevent the unhandled rejection error. It's a big of a hack, but seems to work reliably in testing until we can get upstream to fix the behavior (issue is already opened).