Adds --no-validator CLI flag (#3348)

* Implement `is_validator` for offchain-workers

* Introduce `--no-validator` flag

* Don't run babe/grandpa/im-online when `--no-validator` is given

* Fixes compilation

* Bump spec version

* Improve error handling in executor

* Add missing extern function

* Revert making error public

* Remove `--no-validator` CLI

substrate/core/state-machine/src/lib.rs

@@ -263,6 +263,10 @@ impl NeverOffchainExt {
 }
 
 impl offchain::Externalities for NeverOffchainExt {
+	fn is_validator(&self) -> bool {
+		unreachable!()
+	}
+
 	fn submit_transaction(&mut self, _extrinsic: Vec<u8>) -> Result<(), ()> {
 		unreachable!()
 	}