Move ethcore::Error to error_chain (#8386)

* WIP

* Convert Ethcore error to use error_chain

* Use error_chain for ImportError and BlockImportError

* Fix error pattern matches for error_chain in miner

* Implement explicit From for AccountsError

* Fix pattern matches for ErrorKinds

* Handle ethcore error_chain in light client

* Explicitly define Result type to avoid shadowing

* Fix remaining Error pattern matches

* Fix tab space formatting

* Helps if the tests compile

* Fix error chain matching after merge