Fix: Connection terminated unexpectedly error not caught in on error listener

[rchandranmani]

Our application was crashing when the database connection unexpectedly failed. This happened because:

1. We were previously using removeAllListeners('error'), which unintentionally removed the pg library's error handlers.
2. This left connection termination errors unhandled, leading to app crashes which uses this connector

We've fixed this by:

1. Completely removing the removeAllListeners('error') call within the connector. This ensures that the pg library's built-in error handling for connection issues remains active.
2. We now only add our connector's specific error listener if the pg client doesn't already have any error listeners attached (!pg.listenerCount('error')). This prevents us from creating too many listeners (addressing past MaxEventListeners warnings) while ensuring all critical connection errors are still properly managed.

Fixes https://github.com/loopbackio/loopback-connector-postgresql/issues/778 See also https://github.com/loopbackio/loopback-connector-postgresql/issues/746

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