Display easy to read error messages to the user in Duo Chat

Context

This issue is a follow-up to #426545 (closed) where errors the chat could provide were identified.

Goal

Show users of GitLab Duo Chat helpful, human-readable error messages by updating the current text that error shows. Help them remediate the errors they encounter.

The errors and proposed copy are documented in this google sheet.

We want error messages that are:

1. helpful, in a way that user know what to do to fix the issue - if possible
2. not unhelpful, in a sense that we should not tell the user to do something that we know does not help (e.g. in line 2 we have the question If the prompt is too long, shouldn't the UI catch that?. If the time out is due to a long prompt we should tell the user exactly that, rather asking to be more specific.)
3. unique, so that when users tell us they got this error we can distinguish different errors
4. friendly

Finalized wording for messages to implement

See the green column in this spreadsheet.

Include new documentation

Please see this comment for information about how to document