Make it easier to run without frame introspection

Turning off frame introspection can make things easier to
debug, but will break features like tracebacks.

If we turn it off, instead of trying to collect a traceback and
crashing, just print out a warning and return an empty traceback.

src/codegen/unwinding.cpp

@@ -439,6 +439,15 @@ static const LineInfo* lineInfoForFrame(PythonFrameIterator& frame_it) {
 std::vector<const LineInfo*> getTracebackEntries() {
     std::vector<const LineInfo*> entries;
 
+    if (!ENABLE_FRAME_INTROSPECTION) {
+        static bool printed_warning = false;
+        if (!printed_warning) {
+            printed_warning = true;
+            fprintf(stderr, "Warning: can't get traceback since ENABLE_FRAME_INTROSPECTION=0\n");
+        }
+        return entries;
+    }
+
     for (auto& frame_info : unwindPythonFrames()) {
         const LineInfo* line_info = lineInfoForFrame(frame_info);
         if (line_info)