Misleading error message when submitting empty comment

[mmanela]

I just started playing with this extension and I noticed I am getting a misleading error message when I accidentally tried to add an empty comment. This is a super minor issue but I figured this would be a good reason to download the code and see how it was built. I am sending a PR to fix this.

• Extension version: 0.2.1
• VSCode Version:1.28.1
• OS: Windows

Steps to Reproduce:

1. Add a couple new lines in the comment box
2. Click Add