fixes failure output in mocha init w/o target

[boneskull]

yargs.exit() is not always present, apparently. it will throw in this case--we don't need to be calling it anyway.

before this change, you see the following when running mocha init w/o the <path>

mocha init <path>

create a client-side Mocha setup at <path>

Positional Arguments
  path                                                       [string] [required]

Other Options
  --help, -h     Show usage information & exit                         [boolean]
  --version, -V  Show version number & exit                            [boolean]

✖ ERROR: Not enough non-option arguments: got 0, need at least 1
/Users/boneskull/projects/mochajs/mocha/node_modules/yargs/yargs.js:1172
      else throw err
           ^

TypeError: yargs.exit is not a function
    at Array.<anonymous> (/Users/boneskull/projects/mochajs/mocha/lib/cli/cli.js:55:13)
    at Object.fail (/Users/boneskull/projects/mochajs/mocha/node_modules/yargs/lib/usage.js:41:17)
    at Object.positionalCount (/Users/boneskull/projects/mochajs/mocha/node_modules/yargs/lib/validation.js:51:13)
    at populatePositionals (/Users/boneskull/projects/mochajs/mocha/node_modules/yargs/lib/command.js:301:16)
    at Object.runCommand (/Users/boneskull/projects/mochajs/mocha/node_modules/yargs/lib/command.js:225:23)
    at Object.parseArgs [as _parseArgs] (/Users/boneskull/projects/mochajs/mocha/node_modules/yargs/yargs.js:1087:30)
    at Object.parse (/Users/boneskull/projects/mochajs/mocha/node_modules/yargs/yargs.js:575:25)
    at Object.exports.main (/Users/boneskull/projects/mochajs/mocha/lib/cli/cli.js:71:6)
    at Object.<anonymous> (/Users/boneskull/projects/mochajs/mocha/bin/mocha:153:29)
    at Module._compile (internal/modules/cjs/loader.js:1200:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1220:10)
    at Module.load (internal/modules/cjs/loader.js:1049:32)
    at Function.Module._load (internal/modules/cjs/loader.js:937:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
    at internal/main/run_main_module.js:17:47

[boneskull]

just gonna merge this, probably not a huge deal.