Adding an existence test to files pass to mocha with --file flag (closes #4047)

[ntedgi]

as explained in #4047

Description of the Change

Adding an existence test to files pass to mocha with --file flag
If specify non-existed file with --file option, mocha throw exception.

instead of showing the stack trace

./bin/mocha --file non-existed
/Users/mocha/node_modules/yargs/yargs.js:1163
      else throw err
           ^

Error: Cannot find module '/Users/mocha/non-existed'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:582:15)
    at Function.Module._load (internal/modules/cjs/loader.js:508:25)
    at Module.require (internal/modules/cjs/loader.js:637:17)
    at require (internal/modules/cjs/helpers.js:22:18)
    at /Users/mocha/lib/mocha.js:309:36
    at Array.forEach (<anonymous>)
    at Mocha.loadFiles (/Users/mocha/lib/mocha.js:306:14)
    at Mocha.run (/Users/mocha/lib/mocha.js:784:10)
    at Object.exports.singleRun (/Users/mocha/lib/cli/run-helpers.js:108:16)
    at exports.runMocha (/Users/mocha/lib/cli/run-helpers.js:143:13)
    at Object.exports.handler.argv [as handler] (/Users/mocha/lib/cli/run.js:305:3)
    at Object.runCommand (/Users/mocha/node_modules/yargs/lib/command.js:242:26)
    at Object.parseArgs [as _parseArgs] (/Users/mocha/node_modules/yargs/yargs.js:1104:24)
    at Object.parse (/Users/mocha/node_modules/yargs/yargs.js:566:25)
    at Object.exports.main (/Users/mocha/lib/cli/cli.js:68:6)
    at Object.<anonymous> (/Users/mocha/bin/mocha:131:29)
    at Module._compile (internal/modules/cjs/loader.js:701:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:712:10)
    at Module.load (internal/modules/cjs/loader.js:600:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:539:12)
    at Function.Module._load (internal/modules/cjs/loader.js:531:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:754:12)
    at startup (internal/bootstrap/node.js:283:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:622:3)

Benefits

more friendly an informative way to terminate the process .

[jsf-clabot]

All committers have signed the CLA.

[coveralls]

Coverage increased (+0.07%) to 92.927% when pulling c059efc on ntedgi:naort/issue/4047 into d0f04e9 on mochajs:master.

[boneskull]

Thanks for this!

I agree that it's currently not a great user experience. However, attempting to check the file before trying to require it is a lot of complexity. Can you change the code so, instead, there's a try/catch/rethrow around this line:

mocha/lib/mocha.js

Line 324 in ac25cf7

suite.emit(EVENT_FILE_REQUIRE, require(file), file, self);

[boneskull]

just because the file exists doesn't mean we have permission to stat it. likewise, this will fail on symlinks.