Consider using `if (error instanceof PostgresError)` in `isPgConnectionError`

[zone117x]

The isPgConnectionError function checks for retryable errors using a set of error codes and strings that we've seen so far, and it's not exhaustive (see #36). We should consider checking if the error is an instance of PostgresError at the end of the string matching to catch ones we haven't yet included.

  if (error instanceof PostgresError) {
    return error.message;
  }

Unless there are instances of PostgresError that we want to ignore here?