Surface MultiScan async read failure instead of asserting (facebook#14171)

Summary:
Crash tests have been failing of late with this assertion failure - db_stress: `./table/block_based/block_based_table_iterator.h:656: void rocksdb::BlockBasedTableIterator::PrepareReadAsyncCallBack(rocksdb::FSReadRequest &, void *): Assertion `async_state->status.IsAborted()' failed.` Instead of asserting, surface the failure status so we can troubleshoot.

Pull Request resolved: facebook#14171

Reviewed By: xingbowang

Differential Revision: D88396654

Pulled By: anand1976

fbshipit-source-id: 8d59d7ace0c522c17b7af17c50e16af876911bad

Author: anand1976

table/block_based/block_based_table_iterator.h

@@ -652,8 +652,6 @@ class BlockBasedTableIterator : public InternalIteratorBase<Slice> {
             std::to_string(async_state->offset) + " and async callback " +
             std::to_string(req.offset));
       }
-    } else {
-      assert(async_state->status.IsAborted());
     }
   }