new error type

ethcore/src/engines/authority_round.rs

@@ -275,7 +275,6 @@ impl Engine for AuthorityRound {
 	}
 
 	fn verify_block_family(&self, header: &Header, parent: &Header, _block: Option<&[u8]>) -> Result<(), Error> {
-		// Don't calculate difficulty for genesis blocks.
 		if header.number() == 0 {
 			return Err(From::from(BlockError::RidiculousNumber(OutOfBounds { min: Some(1), max: None, found: header.number() })));
 		}

ethcore/src/engines/mod.rs

@@ -56,6 +56,8 @@ pub enum EngineError {
 	NotProposer(Mismatch<H160>),
 	/// Message was not expected.
 	UnexpectedMessage,
+	/// Seal field has an unexpected size.
+	BadSealFieldSize(OutOfBounds<usize>),
 }
 
 impl fmt::Display for EngineError {
@@ -66,6 +68,7 @@ impl fmt::Display for EngineError {
 			NotProposer(ref mis) => format!("Author is not a current proposer: {}", mis),
 			NotAuthorized(ref address) => format!("Signer {} is not authorized.", address),
 			UnexpectedMessage => "This Engine should not be fed messages.".into(),
+			BadSealFieldSize(ref oob) => format!("Seal field has an unexpected length: {}", oob),
 		};
 
 		f.write_fmt(format_args!("Engine error ({})", msg))