Implement basic import auto-healing for streamlit.

templates.py

@@ -214,8 +214,15 @@ def update_iframe_js(demo_type: DemoType) -> str:
                     }}
                     const newCode = code + ` # Update tag ${{Math.random()}}`;
                     const entrypointFile = "streamlit_app.py";
-                    // TODO: As code rerun happens inside streamlit this won't throw an error for self-healing imports
+                    // As code rerun happens inside streamlit this won't throw an error for self-healing imports.
                     await appController.writeFile(entrypointFile, newCode);
+                    // So instead wait 500 milliseconds to see if the streamlit error banner appeared with an error.
+                    // TODO: Consider a way to make this not rely on streamlit refresh timing; otherwise user can just re-update and this will trigger.
+                    await new Promise(r => setTimeout(r, 500));
+                    const messageDiv = document.getElementById('stlite-iframe').contentWindow.document.querySelector('.message');
+                    if (messageDiv) {{
+                        throw Error(messageDiv.innerHTML);
+                    }}
                 }}
                 catch (e) {{                    
                     // If the error is caused by a missing module try once to install it and update again.