fix(cli): improve no stacks found message in synth output

[nithishravindra]

Related issue

Fixes #2793

Description

Updates the error message shown when no stacks are found during cdktf synth to be more descriptive. This helps users understand they need to define at least one TerraformStack in their CDKTF application

Checklist

• I have updated the PR title to match CDKTF's style guide
• I have run the linter on my code locally
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation if applicable
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works if applicable
• New and existing unit tests pass locally with my changes

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