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