FIX Prevent segfaults in CLI runner

After many hours of debugging, I minimized the problem down to this:
emscripten-core/emscripten#22052

Thanks to @henryiii for reporting. See thread here for my comments while I was
debugging:
scikit-hep/boost-histogram#938

src/core/error_handling.c

@@ -119,13 +119,13 @@ EM_JS(JsVal, restore_stderr, (void), {
 /**
  * Wrap the exception in a JavaScript PythonError object.
  *
- * The return value of this function is always a valid hiwire ID to an error
- * object. It never returns NULL.
+ * The return value of this function is always a JavaScript error object. It
+ * never returns null.
  *
  * We are cautious about leaking the Python stack frame, so we don't increment
  * the reference count on the exception object, we just store a pointer to it.
- * Later we can check if this pointer is equal to sys.last_exc and if so
- * restore the exception (see restore_sys_last_exception).
+ * Later we can check if this pointer is equal to sys.last_exc and if so restore
+ * the exception (see restore_sys_last_exception).
  *
  * WARNING: dereferencing the error pointer stored on the PythonError is a
  * use-after-free bug.

src/js/load-package.ts

@@ -555,6 +555,9 @@ export async function loadPackage(
       Array.from(toLoad.values()).map(({ installPromise }) => installPromise),
     );
 
+    // Warning: this sounds like it might not do anything important, but it
+    // fills in the GOT. There can be segfaults if we leave it out.
+    // See https://github.com/emscripten-core/emscripten/issues/22052
     Module.reportUndefinedSymbols();
     if (loadedPackageData.size > 0) {
       const successNames = Array.from(loadedPackageData, (pkg) => pkg.name)

src/templates/python

@@ -142,6 +142,10 @@ async function main() {
             console.error(e);
         }
     }
+    // Warning: this sounds like it might not do anything important, but it
+    // fills in the GOT. There can be segfaults if we leave it out.
+    // See https://github.com/emscripten-core/emscripten/issues/22052
+    py._module.reportUndefinedSymbols();
 
     py.runPython(
         `
@@ -176,14 +180,10 @@ async function main() {
     try {
         errcode = py._module._run_main();
     } catch (e) {
-        // If someone called exit, just exit with the right return code.
         if(e.constructor.name === "ExitStatus"){
             process.exit(e.status);
         }
-        // Otherwise if there is some sort of error, include the Python
-        // tracebook in addition to the JavaScript traceback
-        py._module._dump_traceback();
-        throw e;
+        py._api.fatal_error(e);
     }
     if (errcode) {
         process.exit(errcode);