* 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
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