feat(CLI): validate drive (#587)

The Etcher CLI doesn't care if the drive exists or not. The user will
eventually find out since the CLI will output an `ENOENT` when trying to
actually write data, however its nicer from a UX perspective to catch
this early on.

Change-Type: minor
Changelog-Entry: Validate the existence of the passed drive.
Signed-off-by: Juan Cruz Viotti <jviottidc@gmail.com>

lib/cli/etcher.js

@@ -16,8 +16,11 @@
 
 'use strict';
 
+const _ = require('lodash');
+const Bluebird = require('bluebird');
 const visuals = require('resin-cli-visuals');
 const form = require('resin-cli-form');
+const drivelist = Bluebird.promisifyAll(require('drivelist'));
 const writer = require('./writer');
 const utils = require('./utils');
 const options = require('./cli');
@@ -56,9 +59,16 @@ form.run([
     check: new visuals.Progress('Validating')
   };
 
-  return writer.writeImage(options._[0], {
+  return drivelist.listAsync().then((drives) => {
+    const selectedDrive = _.find(drives, {
       device: answers.drive
-  }, {
+    });
+
+    if (!selectedDrive) {
+      throw new Error(`Drive not found: ${selectedDrive}`);
+    }
+
+    return writer.writeImage(options._[0], selectedDrive, {
       unmountOnSuccess: options.unmount,
       validateWriteOnSuccess: options.check
     }, (state) => {
@@ -78,6 +88,7 @@ form.run([
       }
 
     });
+  });
 }).then((results) => {
 
   if (options.robot) {