Improve error handling for missing RomM platform after upgrade

[gantoine]

TL;DR

Added a clearer, actionable error message when users encounter missing RomM platform mappings after upgrading from pre-0.6 versions, replacing cryptic "-1 not found" errors with helpful guidance on how to reconfigure their settings.

What changed?

• Split platform validation into two separate checks with distinct error handling:

  • Unconfigured emulator/profile mappings now fail silently (existing behavior)
  • Missing RomM platform selection now triggers a user-facing notification with actionable instructions

• Enhanced the missing platform detection to check both RomMPlatformId <= 0 and RomMPlatform nullability, covering both unset mappings (-1) and empty defaults (0)

• Added a user-friendly error notification that:

  • Uses the mapping name or emulator name for clarity
  • Generates stable notification IDs per mapping to prevent duplicate notifications on repeated imports
  • Explains that this commonly occurs after plugin upgrades
  • Provides step-by-step instructions to resolve the issue

• Improved code comments explaining the upgrade migration scenario and why special handling is needed

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