Disallow any inset inside ERT

Attempting to do this crashes in master, and is not supported anyway.

src/insets/InsetERT.cpp

@@ -188,6 +188,9 @@ bool InsetERT::getStatus(Cursor & cur, FuncRequest const & cmd,
 	case LFUN_INSET_END_EDIT:
 		status.setEnabled(tempfile_ != 0);
 		return true;
+	case LFUN_INSET_INSERT:
+		status.setEnabled(false);
+		return true;
 	case LFUN_INSET_MODIFY:
 		if (cmd.getArg(0) == "ert") {
 			status.setEnabled(true);

src/insets/InsetERT.h

@@ -80,6 +80,8 @@ private:
 	///
 	bool allowSpellCheck() const { return false; }
 	///
+	bool insetAllowed(InsetCode /* code */) const { return false; }
+	///
 	unique_ptr<support::TempFile> tempfile_;
 };