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

js/src/api/transport/error.js

@@ -21,6 +21,7 @@ export const ERROR_CODES = {
   NO_WORK: -32001,
   NO_AUTHOR: -32002,
   NO_NEW_WORK: -32003,
+  NO_WORK_REQUIRED: -32004,
   NOT_ENOUGH_DATA: -32006,
   UNKNOWN_ERROR: -32009,
   TRANSACTION_ERROR: -32010,
@@ -38,7 +39,7 @@ export const ERROR_CODES = {
   COMPILATION_ERROR: -32050,
   ENCRYPTION_ERROR: -32055,
   FETCH_ERROR: -32060,
-  INVALID_PARAMS: -32602
+  INVALID_PARAMS: -32602,
 };
 
 export default class TransportError extends ExtendableError {