[BREAKING] Add error displays for trouble loading model files

[sdatkinson]

Description

Related to #663, will provide more insight on what's going wrong and possibly alert users on how to find a workaround (i.e. place files in a place macOS can grant permission to).

This doesn't resolve the Issue, but should help give information in general.

Breaking change--signature of LoadFileAtCurrentIndex() is altered.

PR Checklist

• Did you format your code using format.bash?
• Does the VST3 plugin pass all of the unit tests in the VST3PluginTestHost? (Download it as part of the VST3 SDK here.)
  • Windows
  • macOS
• [N] Does your PR add, remove, or rename any plugin parameters?
• [N] Does your PR add or remove any graphical assets?

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in more detail...

Summary

• Make file loading explicitly distinguish between files selected through the file picker and existing browser selections used by the arrows or menu.
• Detect the Issue [BUG] Unable to select Models or IRs in Logic Pro 12.3, works on standalone app #663 symptom where a file-picker selection leaves the selected index at -1.
• Show a visible (FAILED) state with a specific directory-enumeration explanation for that case.
• Report other invalid browser-selection states with a generic error instead of incorrectly attributing them to directory permissions.
• Log the load source, selected index, and item count to aid future diagnosis.

This does not attempt to bypass macOS permissions. It prevents a suspected parent-directory scan failure from being silently ignored and distinguishes it from unrelated browser-navigation failures.

Verification

• Built the universal Debug AU successfully for arm64 and x86_64:

  xcodebuild -project NeuralAmpModeler/projects/NeuralAmpModeler-macOS.xcodeproj -target AU -configuration Debug -xcconfig NeuralAmpModeler/config/NeuralAmpModeler-mac.xcconfig CODE_SIGNING_ALLOWED=NO build

• Reproduced the original silent behavior in Logic Pro 12.3 with known-good .nam and .wav files whose exact paths were readable but whose parent directory could not be enumerated.

• Confirmed that the compiled AU contains both the specific file-picker/index--1 diagnostic and the generic unexpected-selection diagnostic.