C#: Handle population of error types

Author: hvitved

csharp/extractor/Semmle.Extraction.CSharp/Entities/Types/Type.cs

@@ -80,6 +80,7 @@ static Kinds.TypeKind GetClassType(Context cx, ITypeSymbol t, bool constructUnde
                         case TypeKind.Enum: return Kinds.TypeKind.ENUM;
                         case TypeKind.Delegate: return Kinds.TypeKind.DELEGATE;
                         case TypeKind.Pointer: return Kinds.TypeKind.POINTER;
+                        case TypeKind.Error: return Kinds.TypeKind.UNKNOWN;
                         default:
                             cx.ModelError(t, $"Unhandled type kind '{t.TypeKind}'");
                             return Kinds.TypeKind.UNKNOWN;