Fix keys file overwrite on installation and method name typo

Summary

Fix key file installation failures being silently skipped.

Problem

When installing keys from a folder, validation failures were silently logged and files were skipped, leaving users with incomplete key installations. As shown in the log file attached to issue #189, the output displays "New Keys file successfully installed" but then firmware installation fails with "Unable to decrypt NCA section" for missing keys.

Root Cause

Invalid key files (e.g., corrupted title.keys) fail VerifyKeysFile() but the error handling silently continues, skipping the file without user notification.

Changes

• Track and report failed key file validations instead of skipping silently
• Simplify file copy: use File.Copy(source, dest, true)
• Fix typo: AreKeysAlredyPresent → AreKeysAlreadyPresent

Fixes #189