[ethash] chainspec validate ecip1017EraRounds non-zero (#11123)

* [ethash]: validate `ecip1017EraRounds` non-zero

When `ecip1017EraRounds` ethash will divide by zero.

This commit ensures that the chainspec deserialization fails and
gives a better error message.

* [ecip1017_eras_block_reward]: document behaviour

* nit

* docs(ethash ecip1071): resolve `TODO`

json/src/spec/ethash.rs

@@ -91,6 +91,7 @@ pub struct EthashParams {
 	pub ecip1010_continue_transition: Option<Uint>,
 
 	/// See main EthashParams docs.
+	#[serde(default, deserialize_with="uint::validate_optional_non_zero")]
 	pub ecip1017_era_rounds: Option<Uint>,
 
 	/// Delays of difficulty bombs.
@@ -101,7 +102,6 @@ pub struct EthashParams {
 	/// EXPIP-2 duration limit
 	pub expip2_duration_limit: Option<Uint>,
 	/// Block to transition to progpow
-	#[serde(rename="progpowTransition")]
 	pub progpow_transition: Option<Uint>,
 }