core: Improve error outputs across core functions

[MickLesk]

✍️ Description

qoL Fixes and Imrovments

• Replaced raw echo -e messages with structured msg_error/msg_warn/msg_info calls across all three core files for consistent formatting.
• Added error checking to previously unchecked critical commands (pveam download, apk add, apt install, systemctl enable).
• Converted silent return 1 / return 0 failure paths into explicit warnings so users actually see what went wrong (e.g. failed API calls, missing tags, repo validation).
• Eliminated || true swallowing on non-trivial operations (composer, mysql upgrades, rustup) — replaced with msg_warn so failures are visible but non-fatal.
• Added context to error messages: URLs, retry counts, storage names, method lists — making diagnosis easier.
• Fixed bare exit calls to use proper exit codes (exit 0 / exit 1).
• Added graceful fallback when error_handler.func download fails inside containers, preventing silent breakage.
• Replaced [DEBUG] raw echos and leftover troubleshooting prints with proper msg_warn/msg_custom calls.

🔗 Related Issue

Fixes #

✅ Prerequisites (X in brackets)

• Self-review completed – Code follows project standards.
• Tested thoroughly – Changes work as expected.
• No security risks – No hardcoded secrets, unnecessary privilege escalations, or permission issues.

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🛠️ Type of Change (X in brackets)

• 🐞 Bug fix – Resolves an issue without breaking functionality.
• ✨ New feature – Adds new, non-breaking functionality.
• 💥 Breaking change – Alters existing functionality in a way that may require updates.
• 🆕 New script – A fully functional and tested script or script set.
• 🌍 Website update – Changes to website-related JSON files or metadata.
• 🔧 Refactoring / Code Cleanup – Improves readability or maintainability without changing functionality.
• 📝 Documentation update – Changes to README, AppName.md, CONTRIBUTING.md, or other docs.