error out explicitly when extra config file passed via -c isnt found

code is duplicated in main action and listRules, but didn't see a really
good way modularize it yet. Perhaps when taking care of #90 or #133 it'll be
more evident?

e2e/test.js

@@ -53,6 +53,24 @@ describe('main executable tests', function () {
     let code
     let stderr
     let stdout
+
+    describe('GIVEN a non-existing extra config file passed with -c WHEN linting', function () {
+      beforeEach(function () {
+        ;({ code, stderr, stdout } = shell.exec('solhint -c nothere.json Foo.sol', {
+          silent: true,
+        }))
+      })
+
+      it('THEN linter exits with error 255 for bad options', function () {
+        expect(code).to.equal(255)
+      })
+      it('AND stdout is empty', function () {
+        expect(stdout.trim()).to.eq('')
+      })
+      it('AND stderr logs the file wasnt found', function () {
+        expect(stderr.trim()).to.include('Extra config file "nothere.json" couldnt be found')
+      })
+    })
     describe('GIVEN a config file with invalid syntax, WHEN linting', function () {
       beforeEach(function () {
         ;({ code, stderr, stdout } = shell.exec('solhint -c broken-json-syntax.json Foo.sol', {
@@ -394,11 +412,13 @@ describe('main executable tests', function () {
             { silent: true }
           ))
         })
-        it('THEN it returns error code 1', function () {
-          expect(code).to.equal(1)
+        it('THEN it returns error code 255 for bad options', function () {
+          expect(code).to.equal(255)
         })
-        it('AND it reports the problem to stderr', function () {
-          expect(stderr).to.contain("Failed to load a solhint's config file")
+        it('AND stderr logs the file wasnt found', function () {
+          expect(stderr.trim()).to.include(
+            'Extra config file "config-file-with-weird-name.json" couldnt be found'
+          )
         })
         it('AND it does NOT list disabled rules', function () {
           expect(stdout).to.eq('')

solhint.js

@@ -71,6 +71,13 @@ function execMainAction() {
     console.error(ex.message)
     process.exit(EXIT_CODES.BAD_OPTIONS)
   }
+
+  const customConfig = rootCommand.opts().config
+  if (customConfig && !fs.existsSync(customConfig)) {
+    console.error(`Extra config file "${customConfig}" couldnt be found.`)
+    process.exit(EXIT_CODES.BAD_OPTIONS)
+  }
+
   let reports
   try {
     const reportLists = rootCommand.args.filter(_.isString).map(processPath)
@@ -216,7 +223,13 @@ function consumeReport(reports, formatterFn) {
 }
 
 function listRules() {
-  const { config } = loadFullConfigurationForPath('.', rootCommand.opts().config)
+  const customConfig = rootCommand.opts().config
+  if (customConfig && !fs.existsSync(customConfig)) {
+    console.error(`Extra config file "${customConfig}" couldnt be found.`)
+    process.exit(EXIT_CODES.BAD_OPTIONS)
+  }
+
+  const { config } = loadFullConfigurationForPath('.', customConfig)
   const rulesObject = config.rules
 
   console.log('\nRules: \n')