Add messaging style to the Zarf style guide and update english.go accordingly

[Racer159]

Describe what should be investigated or refactored

We should include guidance for Zarf messaging in our style-guide as well as our docs pages. There are certain things we would want to call out like:

• New messages should go into a lang file such as english.go.
• Messages should conform to the following:
  • Descriptive name with comment above const block for appropriate go package.
  • If the message is an error message it must have Err in the name, start with a lowercase word and not end in a period if chained to another error (i.e. ends in %s)
  • If the message is an fatal message it must have Fatal in the name, start with an uppercase word and not end in a period if chained to an error (i.e. ends in %s)
  • Regular messages and descriptions should end with punctuation
  • Examples should be contained in parenthesis after the period ending the description

Links to any relevant code

https://github.com/defenseunicorns/zarf/blob/main/docs/12-contribute-to-zarf/4-style-guide.md
https://github.com/defenseunicorns/zarf/blob/main/src/config/lang/english.go

Additional context

@Noxsios mentioned this in a comment here: #1710 (comment)