[UI] [Error] Unable to find image file during export

[jcoliz]

Given: Loaded into UI a TOML file with a logo that points to a non-existing file (see issue-20.toml)
When: Export
Then: (Currently) Fails silently, logs an exception
Expected: Display an error to user

Logs:

[17:30:19 ERR] Export: Failed
System.ApplicationException: Unable to load image smiley.png
 ---> System.IO.FileNotFoundException: Could not find file 'C:\Source\jcoliz\LogoSlideMaker\LogoSlideMaker.WinUi\Sample\smiley.png'.
File name: 'C:\Source\jcoliz\LogoSlideMaker\LogoSlideMaker.WinUi\Sample\smiley.png'
   at Microsoft.Win32.SafeHandles.SafeFileHandle.CreateFile(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options)
   at Microsoft.Win32.SafeHandles.SafeFileHandle.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.Strategies.OSFileStreamStrategy..ctor(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.Strategies.FileStreamHelpers.ChooseStrategyCore(String path, FileMode mode, FileAccess access, FileShare share, FileOptions options, Int64 preallocationSize, Nullable`1 unixCreateMode)
   at System.IO.File.OpenRead(String path)
   at LogoSlideMaker.Export.ImageCache.LoadImageAsync(String filename)
   at LogoSlideMaker.Export.ImageCache.LoadAsync(String path)
   --- End of inner exception stack trace ---
   at LogoSlideMaker.Export.ImageCache.LoadAsync(String path)
   at LogoSlideMaker.Export.ImageCache.LoadAsync(IEnumerable`1 paths)
   at LogoSlideMaker.Export.ExportPipeline.LoadAndMeasureAsync(String basePath)
   at LogoSlideMaker.WinUi.ViewModels.MainViewModel.ExportToAsync(String outPath)
   at LogoSlideMaker.WinUi.MainWindow.DoExport_Click(Object _, RoutedEventArgs __)