Catch exceptions from C++ in EXPLAIN for DXL generation

When generating DXL output in EXPLAIN {ANALYZE}, make sure to catch
any exceptions generated on the C++ side to avoid a server crash on
queries where ORCA fail to generate a plan. A better job of catching
the various different possible exceptions and generating nice error
messages can be made, this closes the current hole of trivial core
dumps on for example this: EXPLAIN (dxl) SELECT * FROM pg_class;

src/backend/gpopt/CGPOptimizer.cpp

@@ -113,7 +113,18 @@ CGPOptimizer::SzDXLPlan
 	Query *pquery
 	)
 {
-	return COptTasks::SzOptimize(pquery);
+	GPOS_TRY;
+	{
+		return COptTasks::SzOptimize(pquery);
+	}
+	GPOS_CATCH_EX(ex);
+	{
+		errstart(ERROR, ex.SzFilename(), ex.UlLine(), NULL, TEXTDOMAIN);
+		errfinish(errcode(ERRCODE_INTERNAL_ERROR),
+				errmsg("Optimizer failed to produce plan"));
+	}
+	GPOS_CATCH_END;
+	return NULL;
 }
 
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