Adds error message to missing file arg

DMCompiler/DMCompiler.cs

@@ -51,18 +51,18 @@
             ForcedWarning("Unimplemented proc & var warnings are currently suppressed");
         }
 
         DMPreprocessor preprocessor = Preprocess(settings.Files, settings.MacroDefines);
         bool successfulCompile = preprocessor is not null && Compile(preprocessor);
 
         if (successfulCompile) {
             //Output file is the first file with the extension changed to .json
             string outputFile = Path.ChangeExtension(settings.Files[0], "json");
             List<DreamMapJson> maps = ConvertMaps(preprocessor.IncludedMaps);
 
             if (ErrorCount > 0) {
                 successfulCompile = false;
             } else {
                 var output = SaveJson(maps, preprocessor.IncludedInterface, outputFile);
                 if (ErrorCount > 0) {
                     successfulCompile = false;
                 } else {
@@ -101,6 +101,11 @@
             // NB: IncludeFile pushes newly seen files to a stack, so push
             // them in reverse order to process them in forward order.
             for (var i = files.Count - 1; i >= 0; i--) {
+                if (!File.Exists(files[i])) {
+                    Console.WriteLine($"'{files[i]}' does not exist");
+                    return null;
+                }
+
                 string includeDir = Path.GetDirectoryName(files[i]);
                 string fileName = Path.GetFileName(files[i]);
 
@@ -133,12 +138,12 @@
         if (Settings.DumpPreprocessor) {
             //Preprocessing is done twice because the output is used up when dumping it
             StringBuilder result = new();
             foreach (Token t in Build()) {
                 result.Append(t.Text);
             }
 
             string outputDir = Path.GetDirectoryName(Settings.Files[0]);
             string outputPath = Path.Combine(outputDir, "preprocessor_dump.dm");
 
             File.WriteAllText(outputPath, result.ToString());
             Console.WriteLine($"Preprocessor output dumped to {outputPath}");