Print explicit error when using an invalid target

[jgiovaresco]

Screenshot:

Closes #87

[jgiovaresco]

I had to confess that I'm not proud of the result... Would you be ok if I make some changes to try to modernize the code?

At least I would like

• use ES6 syntax at least (const, let, arrow...)
• upgrade to Node 8

If you think we should keep compatibility with old node, I can configure Babel to transpile the result 🤔

[jgiovaresco]

@reynolek @develohpanda 🙏

[develohpanda]

Nicely done 💯

use ES6 syntax at least (const, let, arrow...)

That would surely be nice, can be tackled in small chunks.

upgrade to Node 8

But what is the actual change required here? package.json lists node >= 4, so people are free to develop or use whatever version. Also looks like the Dockerfile uses a node 8 base image.

As an aside, because Node 8 will/has been deprecated, I would suggest upgrading to node 10 at the very least, if we do need to do anything here.