references: document snap exit codes

[natibek]

Adds documentation for the snap command's exit codes.

Tracked with SNAPDENG-34132

[Rnfudge02]

Built and tested, looks good. Had a look at references to 'error' and 'exit' in the repo as well, I don't think there's anywhere that really needs to link to this, except I feel the main 'References'[http://127.0.0.1:8000/reference/] page should have a blurb about the exit codes, as every other heading/sub-heading under references seems to be mentioned there. Thoughts?

Does every possible error in snap map to one of the 5 erroring exit status'?

[natibek]

The blurb in the main references page makes sense. I'll add that.

It should cover all the exit codes returned by snap. I double checked by having an agent look for all the exit codes as well and these should be exhaustive.

[Rnfudge02]

Thanks!

[Rnfudge02]

There is a spellcheck for your file 'exit.codes.md'. The word its complaining about is retriable. From what I found that is a valid spelling, but its probably not the american one. Please fix and re run the checks, after that you're good to merge

Retriable (or retryable)

[natibek]

The check is failing for both spellings of retryable. Originally was Retryable, it failed and I changed it to Retriable and it is still failing.

[Rnfudge02]

Okay,

• The tests are passing on main so we shouldn't break them.
• Retryable is the American spelling, and it is a valid word. (change it to this)
• The spellcheck fails even with the American spelling.
• We should find out who maintains the Canonical US spellcheck.
  I'd try to clone tmp locally, figure out what its categorized under?
• I'd see what they say before merging.

I think this may be the best course of action right now, i'm not sure exactly how the admin setup for snap-docs repo is, and I don't have access.