core/state: abort commit if read errors have occurred (#21039)

This finally adds the error check that the documentation of StateDB.dbErr
promises to do. dbErr was added in 9e5f03b6 (June 2017), and the check was
already missing in that commit. We somehow survived without it for three years.
# Conflicts:
#	core/state/statedb.go
#	core/state/statedb_test.go