Add error codes

[JakeDawkins]

This PR adds usable error codes, and associates them with all errors that are handled by Rover.

By usable, I mean there's a new rover explain <CODE> command to explain a single error code

I took inspiration for the explanations themselves being in markdown from https://github.com/rust-lang/rust/blob/master/compiler/rustc_error_codes/src/error_codes.rs

We use termimad to do some basic formatting on the markdown explanations, which also gives us some flexibility to make wrapping tables, etc in our explanations. The only odd thing is that it doesn't support markdown links, so those are just printed as markdown

There are also changes to build.rs that iterate over all files in the codes dir and prints their contents to a errors docs for a reference page

rover explain E001

➜  rover git:(jake/error-codes-coding-errors-code) cargo run -- explain E001
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `target/debug/rover explain E001`
E001

This error occurs when the expected JSON response from a GraphQL endpoint can't
be deserialized.

This is most likely caused by an invalid endpoint or headers, causing the
server to return something that is not JSON (like an HTML error page).

Try running the command again with `--log trace` to see what the GraphQL
endpoint is responding with.

If this error occurs on a command interacting with the Apollo Registry, please
[open an issue](https://github.com/apollographql/rover/issues/new?body=Error%20E001%0A%0ADescribe%20your%20issue%20or%20question%20here&labels=triage)
and let us know!

[EverlastingBugstopper]

This is looking good! I really like the idea of having the errors be in markdown so that we can have some error reference pages in the docs (perfect for SEO on running into errors!) Agree that for printing them out maybe we should try pretty printing the markdown :)

[JakeDawkins]

Since termimad doesn't support markdown links, I decided to replace them with... regex 😬 Let me know if this is terrible, and we can figure out something else.

I'm replacing the markdown urls like [this](https://google.com) with this (https://google.com) to make them a little more readable. I'm also painting them cyan right now to indicate they're urls and to give a little visual separation.

[EverlastingBugstopper]

looking good!

[lrlna]

This is in a really good shape. I absolutely love the rover explain command, this will be so handy!

Left a few comments about the Code API which may perhaps help with its usage.

[lrlna]

LGTM!

[EverlastingBugstopper]

should we maybe error here instead?

[JakeDawkins]

We definitely could 🤷‍♂️ Especially since this is at buildtime. I just left it as the simpler option in the meantime. Do you think I should change that?

[EverlastingBugstopper]

yeah since it's at build time i think maybe we should error here so we don't commit something that doesn't work, i imagine filtering out the errors could end up with some head scratches if something goes wrong

[lrlna]

I am going to keep this as is for now and we can get it to error in an xtask refactor.