Add note about auto-updates when cache is missing

[dbrgn]

Output:

Fixes #250.

@niklasmohrin @uhthomas what do you think?

[niklasmohrin]

To be honest, I think it is a bit verbose and might be more intimidating than helpful for first-time users. How do you fell about:

Error: Cache not found. Please run `tldr --update` to download the cache.

Your first time using tealdeer? You can find some tips and tricks to get started at
    https://dbrgn.github.io/tealdeer/getting_started.html

... and then we put the text you have here in a subsection of configuration (or it already exists, I don't know). In the getting-started page, we could put some links and maybe some config recommendations for ease of use (like auto-update). What do you think?

(I also changed the "Warning" to "Error", because this is an error; I think we should change this on main :D)

[dbrgn]

I thought about it again, and I'd stick to the current version of the PR (with slight rewording). The message will only be shown when a cache is missing, meaning that tealdeer wasn't yet initialized on that system (so usually just once). In this case, it's good if the information is right there, without having to open the webbrowser and visit a website first. (I think I wouldn't do that myself, I'd just copy-paste the command and miss the bit about auto updates.)

Here's an updated version:

@niklasmohrin can you live with that? 🙂

[dbrgn]

Ah, and I changed the warning to an error.

[niklasmohrin]

Code looks good; I personally still think that the note is a bit long and the error is a bit lost in all of the text, but that may just be me (one other person I asked said it's clear to them)