Verbose and error if main is not defined

src/compiler.js

@@ -62,8 +62,13 @@ async function compile(srcFile) {
         fileName: fullFileName
     };
 
+
     exec(ctx, ast);
 
+    if (!ctx.components["main"]) {
+        throw new Error("A main component must be defined");
+    }
+
     classifySignals(ctx);
     reduceConstants(ctx);