[commands] Correct ExtensionNotFound error message

[Soheab]

Summary

I noticed that attempting to reload a nonexistent extension resulted in the library raising an ExtensionNotFound error with the message "Extension 'foo' could not be loaded." This phrasing might be misinterpreted by some as indicating a loading failure, when in fact it denotes that the extension was not found. This PR refines the message for improved clarity.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)