Fix crash when mining with empty keypool.

[domob1812]

Since the introduction of the ScriptForMining callback, the mining functions (setgenerate and generate) crash with an assertion failure (due to a NULL pointer script returned) if the keypool is empty. Fix this by giving a proper error.

[jonasschnelli]

This could also be triggered if the connected signal listener does not provide a script for other reasons than a non-filled keypool. But yes, right now only a empty keypool would emit this exception.

[jonasschnelli]

utACK.
In terms of modularity, the miner should not know the concept of "keypools" and therefore throw an generic exception with a possible error string passed over from the signal listener to the signal emitting part.

[domob1812]

I agree in principle (that's why the message/error is keypool specific only in the RPC code and not miner.cpp). However, the keypool is explicitly a "concept" part of the RPC interface (with the specific error code), thus I think it should be handled there.

Of course, if in the future more conditions can cause a missing script to be returned, one also needs to add a method to signal the error to the caller with more details.

[laanwj]

This has been broken and fixed multiple times. Thanks a lot for adding a test.
ACK