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.)
Could reword it to "To use bitcoind, or 'bitcoin-qt -server'", to use fewer and perhaps clearer words. (That's assuming that the binary can be invoked as bitcoin-qt on all platforms; I don't know how you invoke it on windows. I don't know that it makes sense to say Bitcoin Core Gui, since anybody who's passing commandline arguments to it will be familiar with the name of the binary.)

(I'm assuming the nit was directed purely at gmaxwell; the -server option to bitcoind isn't relevant to the wording of this error message, I think.)