Decoding headers can fail (#8570)

* rlp::decode returns Result

* Fix journaldb to handle rlp::decode Result

* Fix ethcore to work with rlp::decode returning Result

* Light client handles rlp::decode returning Result

* Fix tests in rlp_derive

* Fix tests

* Cleanup

* cleanup

* Allow panic rather than breaking out of iterator

* Let decoding failures when reading from disk blow up

* syntax

* Fix the trivial grumbles

* Fix failing tests

* Make Account::from_rlp return Result

* Syntx, sigh

* Temp-fix for decoding failures

* Header::decode returns Result

Handle new return type throughout the code base.

* Do not continue reading from the DB when a value could not be read

* Fix tests

* Handle header decoding in light_sync

* Handling header decoding errors

* Let the DecodeError bubble up unchanged

* Remove redundant error conversion