Add pgPool 'error' handler

[madflow]

• PgPools don't have error handlers #22
• The process does not exit anymore in case of a backend restart - but there is a Promise Rejection Warning:

1|worker  | (node:64357) UnhandledPromiseRejectionWarning: Error: Client has encountered a connection error and is not queryable
1|worker  |     at process.nextTick (.../node_modules/pg/lib/client.js:521:25)
1|worker  |     at process._tickCallback (internal/process/next_tick.js:61:11)
1|worker  | (node:64357) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
1|worker  | (node:64357) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code

• Maybe there is more work to do here...

[benjie]

Great; thanks for this! 🙏