Improve error message when func invoke is run outside function directory

[RayyanSeliya]

Changes

• 🧹 Update error message shown when func invoke is run outside a function directory.
• Made the message more user-friendly and easier for beginners to understand.
• Added step-by-step workflow guidance with examples for both new and existing functions.

/kind enhancement

Fixes #3017

Release Note

Improve error message when `func invoke` is run outside function directory
to be more beginner-friendly, include step-by-step workflow guidance.

[knative-prow]

Hi @RayyanSeliya. Thanks for your PR.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.40%. Comparing base (ec7ff7e) to head (26eba5e).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/invoke.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3018      +/-   ##
==========================================
+ Coverage   57.67%   59.40%   +1.72%     
==========================================
  Files         133      133              
  Lines       17030    17030              
==========================================
+ Hits         9822    10116     +294     
+ Misses       6305     5954     -351     
- Partials      903      960      +57     

Flag	Coverage Δ
e2e-tests	40.76% <0.00%> (+7.58%)	⬆️
integration-tests	54.10% <0.00%> (+1.63%)	⬆️
unit-tests	46.31% <0.00%> (ø)

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[lkingland]

/ok-to-test

[lkingland]

/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lkingland, RayyanSeliya

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