fix(terraform): improve error message for terraform plan failure

[ramya18101]

Problem

When hc-install fails to install Terraform (e.g. due to an expired OpenPGP key: unable to verify checksums signature: openpgp: invalid message: openpgp: key expired), the error was silently discarded. The user saw only a generic message:

"Terraform resource config generated successfully but there was an error with terraform plan."

This was misleading on two counts:

1. The error came from the install step, not from terraform plan.
2. The actual error message was never shown, leaving users with no actionable information.

🔧 Changes

Include the real error in the warning output so users can diagnose what went wrong — whether it's an OpenPGP verification failure, a network error, a version resolution issue, or anything else from hc-install or tfexec.

📚 References

#1499 (comment)

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.32%. Comparing base (ff0b1b9) to head (24ae536).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/cli/terraform.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1549      +/-   ##
==========================================
- Coverage   59.33%   59.32%   -0.01%     
==========================================
  Files         121      121              
  Lines       23237    23240       +3     
==========================================
  Hits        13787    13787              
- Misses       8576     8579       +3     
  Partials      874      874              

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