Error messages format

Hi!

First of all, congratulations for this release. This package has all the xaringan related functions I've been wanting for.

I just have a suggestion regarding the format of the error messages. I guess some of them are quite easy to understand for developers but they may not be to the wide public to which I'm sure this package is aimed.

In my first contact with the package I tried to compile the template obtained from xaringanthemer to PDF.

> library(xaringanBuilder)
> build_pdf("myslides.html")
✓ Building slides.pdf from slides.html ... done
Error in pagedown::chrome_print(input = input, output = output_file) : 
  The browser is not executable: /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

The error was obviously because I didn't have Google Chrome installed. Maybe a message indicating that this browser must be installed should be given.

I installed it and then get:

> build_pdf("myslides.html")
✓ Building slides.pdf from slides.html ... done
Error in is_remote_protocol_ok(debug_port, verbose = verbose) : 
  Cannot find headless Chrome after 20 attempts

I was executing the code before opening Google Chrome (again, if this is necessary maybe it should be shown in a message). After opening Google Chrome I executed the code again. The slides were correct but I got an error though (which this time I could not understand).

> build_pdf("myslides.html")
✓ Building slides.pdf from slides.html ... done
[2021-02-18 09:58:56] [error] handle_read_frame error: asio.system:54 (Connection reset by peer)

One of the slides runs out of the frame (it also does in the html version). May that be the reason?

Start at the build_pdf() entry point and reproduce the reported commands with pagedown::chrome_print. Review how missing Chrome, Chrome startup, and the connection-reset error are surfaced. Done means the relevant failures provide actionable guidance, including whether Chrome must be installed or opened.