RPCs in blockchain.cpp were returning misleading or incorrect error
codes (for example getblock() returning RPC_INTERNAL_ERROR when the
block had been pruned). This commit fixes those error codes:
- RPC_INTERNAL_ERROR should not be returned for application-level
errors, only for genuine internal errors such as corrupted data.
- RPC_METHOD_NOT_FOUND should not be returned in response to a
JSON request for an existing method.
Those error codes have been replaced with RPC_MISC_ERROR or
RPC_INVALID_PARAMETER as appropriate.
c1190963b3