Add guard to warn user if no serial ports are found

[Brunius]

This is a simple UX change to catch the low hanging fruit of "why doesn't this work? I hit the button and nothing happens!".

Before - Click the button, and this is what you see (no change)

After - Click the button, and an alert pops up!

So far, I have only tested this in Firefox and Edge. Edge will always offer you the option of COM1 on Windows, so it may be necessary to test this on another OS with a Chromium 89+ browser and no serial ports to confirm full coverage.

This impacts on some of the same areas as issue #42 - it's not a fix for operations in non-Chromium browsers, but it should at least let people know what's going on.

[Brunius]

• Could / should this be a more fleshed out error message?
  Yeah, possibly. At this point I wanted something simple and easy - I'm no JS expert, so I just went for the easiest way of doing so.

• We're pointing them to the documentation - should we include a link, or specific section in the documentation?
  Yeah, maybe. I didn't want to play with the documentation too much as I don't know if I'd be treading on any toes (or, where I could play with it). The other aspect to that, is how do we explain to someone who may not know browsers why they need to use a different browser? Or is it enough to simply say "use Edge or Chrome instead of Firefox"?

[Brunius]

Tested on Firefox 114.0.1 (64-bit) and Edge 114.0.1823.43 (64-bit) on Windows 10, with no issues as far as I could see