State sync unexpectedly fail

[bowenwang1996]

On betanet we observe that state sync fail with

Jul 26 14:33:54.650 ERROR sync: Sync: Unexpected error: DB Not Found Error: 3fqj33NedbEA4fMLfVxU7odK2bouHuRgFBRhryAc1Drm
 Cause: Unknown
 Backtrace:    0: failure::backtrace::Backtrace::new
   1: <near_chain::error::Error as core::convert::From<near_primitives::errors::EpochError>>::from
   2: <neard::runtime::NightshadeRuntime as near_chain::types::RuntimeAdapter>::get_epoch_id_from_prev_block
   3: near_client::sync::StateSync::request_shard
   4: near_client::sync::StateSync::sync_shards_status
   5: near_client::sync::StateSync::run
   6: near_client::client_actor::ClientActor::sync
   7: <F as actix::utils::TimerFuncBox<A>>::call
   8: <actix::utils::TimerFunc<A> as actix::fut::ActorFuture>::poll
   9: <actix::contextimpl::ContextFut<A,C> as core::future::future::Future>::poll
  10: tokio::runtime::task::raw::poll
  11: tokio::task::local::LocalSet::tick
  12: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
  13: actix_rt::runtime::Runtime::block_on
  14: near::main
  15: std::rt::lang_start_internal::{{closure}}::{{closure}}
             at src/libstd/rt.rs:52
      std::sys_common::backtrace::__rust_begin_short_backtrace
             at src/libstd/sys_common/backtrace.rs:130
  16: main
  17: __libc_start_main
  18: _start

It appears that information needed for state sync is already garbage collected.