Throw an error if grammer js file is not provided

Currently the error for missing arguments is obscure:

internal/validators.js:120
    throw new ERR_INVALID_ARG_TYPE(name, 'string', value);
    ^

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined
    at validateString (internal/validators.js:120:11)
    at Object.resolve (path.js:980:7)
    at Object.<anonymous> (/home/ali/projects/oblong/node_modules/nearley/bin/nearley-test.js:23:32)
    at Module._compile (internal/modules/cjs/loader.js:1137:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1157:10)
    at Module.load (internal/modules/cjs/loader.js:985:32)
    at Function.Module._load (internal/modules/cjs/loader.js:878:14)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12)
    at internal/main/run_main_module.js:17:47 {
  code: 'ERR_INVALID_ARG_TYPE'
}
error Command failed with exit code 1.

This change makes the resulting error a little more understandable.

kach / nearley

Throw an error if grammer js file is not provided #533