Add "file not found" to the ErrReadingManifest error message to make it clearer that the file doesn't exist, rather than a parsing or permission error.

Change summary

The current error message "error reading fastly.toml" is somewhat ambiguous and doesn't clearly indicate whether the file is missing or has parsing errors. The remediation text was present but it still seems easy to misinterpret the actual problem (as proven by it managing to throw both me, Claude and ChatGPT off 😅).

This change adds ": file not found" to the error message to make it even more apparent that the file doesn't exist.

Before:
ERROR: error reading fastly.toml. Ensure the Fastly CLI is being run within a directory containing a fastly.toml
file...

After:
ERROR: error reading fastly.toml: file not found. Ensure the Fastly CLI is being run within a directory containing
a fastly.toml file...

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for
  the same update/change?

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

• Users will now more easily understand that the fastly.toml file is missing.