Improve error message on missing rpc arguments

[dagurval]

Shows usage instead of assertion error.

[dagurval]

Before:

$ ./bin/bcoin-cli rpc
AssertionError [ERR_ASSERTION]: Assertion failed.
    at NodeClient.execute (/home/dagurval/bclient/node_modules/bcurl/lib/client.js:279:5)
    at NodeClient.execute (/home/dagurval/bclient/lib/node.js:44:18)
    at CLI.rpc (/home/dagurval/bclient/bin/bcoin-cli:159:34)
    at CLI.open (/home/dagurval/bclient/bin/bcoin-cli:195:20)
    at /home/dagurval/bclient/bin/bcoin-cli:221:13
    at Object.<anonymous> (/home/dagurval/bclient/bin/bcoin-cli:223:3)
    at Module._compile (module.js:652:30)
    at Object.Module._extensions..js (module.js:663:10)
    at Module.load (module.js:565:32)
    at tryModuleLoad (module.js:505:12)

After:

$ ./bin/bcoin-cli rpc
Missing arguments
Commands:
  $ info: Get server info.
  $ broadcast [tx-hex]: Broadcast transaction.
  $ mempool: Get mempool snapshot.
  $ tx [hash/address]: View transactions.
  $ coin [hash+index/address]: View coins.
  $ block [hash/height]: View block.
  $ reset [height/hash]: Reset chain to desired block.
  $ rpc [command] [args]: Execute RPC command. (`bcoin-cli rpc help` for more)

[pinheadmz]

This doesn't look quite right, a bad rpc call shouldn't list the REST API commands, it should list the possible rpc commands. Funny you should submit this today, there's ongoing discussion about the missing rpc help here: bcoin-org/bcoin#705

And an older approach not merged yet: bcoin-org/bcoin#550

[dagurval]

I think that's a separate issue. This PR is for letting the user know that he used the cli interface incorrectly (missing arguments). It's an improvement over asserting.

I do agree that bcoin-cli rpc help could use improvement also.

[pinheadmz]

I hear ya, but I think the best UX is providing the user with the list of possible arguments that could be missing. See #20 which in the case of a missing rpc method, will instead call rpc help