fix: Exit with code 1 when nyc doesn't know what to do.

When nyc is not told to do anything this is an error.  We already
printed the help message to stderr, now we exit with code 1 to indicate
that nyc exited without doing anything.

Add a test to cover this edge case and verify that both `nyc` and `nyc
--help` produce output as expected to stdout and stderr, exit with the
proper code.

bin/nyc.js

@@ -85,5 +85,6 @@ if ([
   })
 } else {
   // I don't have a clue what you're doing.
+  process.exitCode = 1
   yargs.showHelp()
 }

test/nyc-integration.js

@@ -1079,6 +1079,40 @@ describe('the nyc cli', function () {
     })
   })
 
+  it('help shows to stderr when main command doesn\'t know what to do', done => {
+    const msgs = {}
+    let waitingFor = 2
+    const doSpawn = (args, type, expectCode) => {
+      const proc = spawn(process.execPath, args, {
+        cwd: fixturesCLI,
+        env
+      })
+      msgs[type] = {
+        out: '',
+        err: ''
+      }
+      proc.stdout.on('data', chunk => {
+        msgs[type].out += chunk
+      })
+      proc.stderr.on('data', chunk => {
+        msgs[type].err += chunk
+      })
+      proc.on('close', code => {
+        code.should.equal(expectCode)
+        waitingFor--
+        if (waitingFor === 0) {
+          msgs.help.out.should.equal(msgs.unknown.err)
+          msgs.help.err.should.equal('')
+          msgs.unknown.out.should.equal('')
+          done()
+        }
+      })
+    }
+
+    doSpawn([bin, '--help'], 'help', 0)
+    doSpawn([bin], 'unknown', 1)
+  })
+
   describe('args', function () {
     it('does not interpret args intended for instrumented bin', function (done) {
       var args = [bin, '--silent', process.execPath, 'args.js', '--help', '--version']