bitcoingui: show main window (if hidden) on modal messages
RPC: error code changes and prevent crash with walletpassphrase

- fix crash with walletpassphrase by checking if RPC server is running and
  give a friendly error message how to fix this (fixes #3100)
- add 3 new RPCErrorCodes RPC_SERVER_NOT_STARTED, RPC_NODE_ALREADY_ADDED
  and RCP_NODE_NOT_ADDED (I checked the source to not use a number already
  in use for RPC_SERVER_NOT_STARTED)
- use the new codes where needed / missing
- add missing use of RPC_INVALID_PARAMETER
[Qt]: fix num Blocks to maturity in Tx description

- as we have main.h included in transactiondesc.cpp, we now also use
  COINBASE_MATURITY +  1 for the descriptive string
- fixes #3131
Merge pull request #3137 from Diapolo/RPC

RPC: error code changes and prevent crash with walletpassphrase
Merge pull request #3139 from Diapolo/maturity

[Qt]: fix num Blocks to maturity in Tx description
Merge pull request #3135 from Diapolo/bitcoingui

bitcoingui: show main window (if hidden) on modal messages