Remove excessive warning

[grbIzl]

This warning is a excessive, because calling sign method of EngineSigner with incorrect password is legit situation. For example, it's being done on the start in order to define proper engine-signer account (all passwords are looped through and tried for signing test message, as result warnings about all incorrect passwords are shown in the console).
Also unifies the logic of EngineSigner because all other components in this layer (rpc/v1/helpers) silently return the result or error.

Relates to #10695

[dvdplm]

Did you git blame to see why it was added in the first place? Also: would s/warn!/trace!/ improve things?

[grbIzl]

Did you git blame to see why it was added in the first place?

Yes, indeed. This warning was introduced by @tomusdrw in his #10213 Before it we tried to sign test message using direct sign call from AccountProvider:

in run.rs:
...
if !passwords.iter().any(|p| miner.set_author(engine_signer, Some(p.to_owned())).is_ok()) {
return Err(format!("No valid password for the consensus signer {}. {}", engine_signer,
VERIFY_PASSWORD_HINT));
...
in miner.rs:
...
fn set_author(&self, address: Address, password: Option) -> Result<(), AccountError> {
self.params.write().author = address;

if self.engine.seals_internally().is_some() && password.is_some() {
	if let Some(ref ap) = self.accounts {
		let password = password.unwrap_or_else(|| Password::from(String::new()));
		// Sign test message
		ap.sign(address.clone(), Some(password.clone()), Default::default())?;

...

@tomusdrw created EngineSigner in RPC in order to communicate with accounts through it and used its sign. I don't know, why he added a global warning in sign method. All account provider methods and RPC helpers just return error and let the caller log it if needed.

Also: would s/warn!/trace!/ improve things?

Obviously it would, but IMO it doesn't make sense for such component to log anything. See before, IMO the user of this component should take care about logging the error. And we have such approach for all other components.

[tomusdrw]

Looks good, but can we make sure we have a warning when we actually try to use EngineSigner from within Engine when new blocks are being sealed?

[grbIzl]

Looks good, but can we make sure we have a warning when we actually try to use EngineSigner from within Engine when new blocks are being sealed?

You mean this warning: https://github.com/paritytech/parity-ethereum/blob/master/ethcore/src/engines/authority_round/mod.rs#L1196 ?

[jam10o-new]

Added patch labels and bumping milestone because this missed being backported