Editor exception thrown from filesystem provider if source file doesn't exist

Start with FilesCompletionProvider.GetEntries in the VSIX stack trace, then inspect FileSystemCatalog.GetLibraryAsync for the missing-source case. Reproduce with the provided libman.json and edit in Visual Studio. Done means editing no longer throws an editor exception, while restore still reports an appropriate error.

Functional impact

When editing libman.json in Visual Studio, if there is a library entry using the filesystem provider, and the file doesn't exist, it will throw an exception in the editor.

Minimal repro steps

1. Have a project with the following libman.json:

{
  "version": "1.0",
  "defaultProvider": "filesystem",
  "libraries": [
    {
      "provider": "filesystem",
      "library": "node_modules/jquery/dist/",
      "destination": "wwwroot/lib/",
      "files": [ 
        "jquery.min.js" 
      ]
    }
  ]
}

2. Make sure the source files (in the node_modules folder) do not exist
3. Make an edit in the editor

Expected result

Edits can happen fine, a restore will fail with an appropriate error

Actual result

As soon as I make an edit (e.g. type any character), I get an error from VS and an entry in the ActivityLog:

System.AggregateException: One or more errors occurred. ---> Microsoft.Web.LibraryManager.Contracts.InvalidLibraryException: The "node_modules/jquery/dist/" library could not be resolved by the "filesystem" provider at Microsoft.Web.LibraryManager.Providers.FileSystem.FileSystemCatalog.d__4.MoveNext() --- End of stack trace from previous location where exception was thrown --- at Microsoft.VisualStudio.Telemetry.WindowsErrorReporting.WatsonReport.GetClrWatsonExceptionInfo(Exception exceptionObject) --- End of inner exception stack trace --- at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions) at System.Threading.Tasks.Task1.GetResultCore(Boolean waitCompletionNotification) at System.Threading.Tasks.Task1.get_Result() at Microsoft.Web.LibraryManager.Vsix.FilesCompletionProvider.d__2.MoveNext() at System.Collections.Generic.List1.InsertRange(Int32 index, IEnumerable1 collection) at Microsoft.JSON.Editor.Completion.JSONCompletionSource.<>c__DisplayClass3_2.b__0() at Microsoft.Web.Editor.Utility.GuardedOperations.InvokeExtensionPoint(Object errorSource, Action action) ---> (Inner Exception #0) Microsoft.Web.LibraryManager.Contracts.InvalidLibraryException: The "node_modules/jquery/dist/" library could not be resolved by the "filesystem" provider at Microsoft.Web.LibraryManager.Providers.FileSystem.FileSystemCatalog.d__4.MoveNext() --- End of stack trace from previous location where exception was thrown --- at Microsoft.VisualStudio.Telemetry.WindowsErrorReporting.WatsonReport.GetClrWatsonExceptionInfo(Exception exceptionObject)<---
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