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

ethcore/src/error.rs

@@ -290,6 +290,12 @@ error_chain! {
 			description("Unknown engine name")
 			display("Unknown engine name ({})", name)
 		}
+
+		#[doc = "RLP decoding errors"]
+		Decoder(err: ::rlp::DecoderError) {
+			description("decoding value failed")
+			display("decoding value failed with error: {}", err)
+		}
 	}
 }
 
@@ -310,11 +316,11 @@ impl From<AccountsError> for Error {
 	fn from(err: AccountsError) -> Error { 
 		ErrorKind::AccountProvider(err).into()
 	} 
-} 
+}
 
 impl From<::rlp::DecoderError> for Error {
 	fn from(err: ::rlp::DecoderError) -> Error {
-		UtilError::from(err).into()
+		ErrorKind::Decoder(err).into()
 	}
 }