Errors & warnings for inappropriate RPCs (#6029)

* Clarify function name

Function checks if sealing is currently underway, not to be confused
with checking whether the engine performs internal sealing.

* Error when work called on internal sealing engine

* Error submitting work for internal sealing engine

* Fix inverted bool and style grumbles

* Add can_produce_work_package to TestMinerService

* Error when setting engine signer on PoW chain

* Unit tests for engine signing

Setting engine signer should fail if chain does not seal internally
or client lacks account provider.

* Tweak TestMinerService

* Fix minor style grumbles

ethcore/src/spec/spec.rs

@@ -451,6 +451,9 @@ impl Spec {
 	/// Create a new Spec with BasicAuthority which uses multiple validator sets changing with height.
 	/// Account with secrets "0".sha3() is the validator for block 1 and with "1".sha3() onwards.
 	pub fn new_validator_multi() -> Self { load_bundled!("validator_multi") }
+
+	/// Create a new spec for a PoW chain
+	pub fn new_pow_test_spec() -> Self { load_bundled!("ethereum/olympic") }
 }
 
 #[cfg(test)]