Better error, better help when command is missing

[pinheadmz]

Both of these easy-to-make mistakes for new users need to provide better answers:

# hsd-rpc
AssertionError [ERR_ASSERTION]: Assertion failed.
    at NodeClient.execute (/root/n/lib/node_modules/hs-client/node_modules/bcurl/lib/client.js:279:5)
    at NodeClient.execute (/root/n/lib/node_modules/hs-client/lib/node.js:44:18)
    at CLI.rpc (/root/n/lib/node_modules/hs-client/bin/hsd-cli:159:34)
    at CLI.open (/root/n/lib/node_modules/hs-client/bin/hsd-cli:195:20)
    at /root/n/lib/node_modules/hs-client/bin/hsd-cli:220:13
    at Object.<anonymous> (/root/n/lib/node_modules/hs-client/bin/hsd-cli:222:3)
    at Module._compile (internal/modules/cjs/loader.js:959:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:995:10)
    at Module.load (internal/modules/cjs/loader.js:815:32)
    at Function.Module._load (internal/modules/cjs/loader.js:727:14)


# hsd-rpc help
Select a command.