This moves the verification of the proof at node startup from init.cpp
to processor.cpp. As a result, the init error message is a bit less
precise, but an additional log line is added to specify the exact cause
of the failure.
`Processor::getProof` was initially meant to be used in abandoned D9327,
so it is currently not needed and is replaced by a simpler `Processor::hasLocalProof`
method that returns a boolean.