C#: Exclude not existing or problematic files from extraction

csharp/extractor/Semmle.Extraction.CSharp.DependencyFetching/DependencyManager.cs

@@ -439,6 +439,25 @@ namespace Semmle.Extraction.CSharp.DependencyFetching
                 files = files.Where(f => !f.FullName.StartsWith(options.DotNetPath, StringComparison.OrdinalIgnoreCase));
             }
 
+            files = files.Where(f =>
+            {
+                try
+                {
+                    if (f.Exists)
+                    {
+                        return true;
+                    }
+
+                    progressMonitor.Log(Severity.Warning, $"File {f.FullName} could not be processed.");
+                    return false;
+                }
+                catch (Exception ex)
+                {
+                    progressMonitor.Log(Severity.Warning, $"File {f.FullName} could not be processed: {ex.Message}");
+                    return false;
+                }
+            });
+
             files = new FilePathFilter(sourceDir, progressMonitor).Filter(files);
             return files;
         }