Truthier error message when rpcpassword is missing

See #5178 -- this change makes the error slightly more honest / less confusing. The message was always being displayed as "To use the -server option". The check for "-server" did not work as intended, because SoftSetBoolArg is used to enable -server anytime we run as bitcoind, so there's no way for this code to tell whether -server was actually passed by the user or not.

If there's some nice way to tell whether we're bitcoind or bitcoin-core, we could customize the error message accordingly; but it's probably not worth adding code in order to do that, so I've made a single error message that covers both.

I have removed all references to -daemon because as far as I can tell that flag has no effect on whether RPC is enabled or not. (RPC-enabling happens only on init.cpp:768, as far as I can tell, and is conditional only on whether -server is set, which happens if the user passes "-server", or if we are bitcoind. I couldn't find any way for -daemon to affect this.)

Previously, one of the possible messages was localized and the other was not. I have left the message localized, but I don't know what else I might need to touch since I changed it. (gmaxwell opined to me that it's better not to localize error messages at all; I'm happy to remove that here if it's undesired.)
killall -SIGINT bitcoind shuts down cleanly, afaik.