Handle duplicate_table errors in system_monitor_pg

[dszoboszlay]

The query creating new table partitions in system_monitor_pg may crash with duplicate_table error in case multiple nodes are using the same DB:

system_monitor/src/system_monitor_pg.erl

Line 180 in 1978c2b

[{ok, [], []}, {ok, [], []}] = epgsql:squery(Conn, Query)

This was surprising to me, because I was expecting IF NOT EXISTS to avoid this error, but as I've learned on Stackoverflow:

The IF NOT EXISTS is meant to deal idempotency, not concurrency.

🤷