Exception: PG::PoolExhausted
Overview
Pool-side error (no SQLSTATE): raised by PG::Pool#checkout when
the pool stays empty past the checkout timeout. Subclasses Error
so callers can rescue PG::PoolExhausted or the broader
rescue PG::Error. (Raising namespaced errors from instance
methods became viable with matz/spinel#1041; before that, checkout
surfaced exhaustion as a sentinel nil-equivalent Connection.)