eth: return iterator errors from debug_storageRangeAt

[ghwo336]

debug_storageRangeAt stops iterating when the iterator's Next method
returns false. Next returns false both when the range is exhausted and
when a trie node cannot be resolved, storing the failure in the
iterator's Err field. The method does not check Err after iteration, so
a trie failure can be reported as a normal result.

This also gives the response the wrong pagination meaning. When
iteration fails, the follow-up Next call used to compute NextKey returns
false and leaves NextKey nil. StorageRangeResult defines a nil NextKey
as meaning that the response includes the last key in the trie. The
caller is therefore told that the range is complete even though it was
truncated by a trie error.

Return the error already produced by the iterator. This follows the
existing handling used by other trie.NewIterator consumers, including
cmd/geth/dbcmd.go and the generators in core/state/snapshot and
triedb/pathdb.

Added a regression test that removes a storage trie node from the
database and verifies that storageRangeAt returns an error instead of a
truncated result.

[rjl493456442]

Pls remove the attached test.

[ghwo336]

Done, removed the test.