Rephrase RPC error messages

I tried to leave out low level details.
A few adjustments were made for consistency.
Indentations were low hanging fruits (based on surrounding code, not overall)
Add error codes of populateRPCTransactionObject
Replace numbers by error codes in populateRPCTransactionObject
Merge pull request #181 from dexX7/mcore-rpc-refine-errors-add-codes

Refine error messages, add codes to populateRPCTransactionObject