Don't throw additional Error in require_once if exception already thrown

As pointed out in comments on bug #66216.

Author: nikic

Zend/tests/require_once_warning_to_exception.phpt

@@ -0,0 +1,19 @@
+--TEST--
+Promoting require_once warning to exception
+--FILE--
+<?php
+
+function exception_error_handler($errno, $errstr, $errfile, $errline ) {
+    throw new Exception($errstr);
+}
+set_error_handler("exception_error_handler");
+
+try {
+    $results = require_once 'does-not-exist.php';
+} catch (Exception $e) {
+    echo $e->getMessage(), "\n";
+};
+
+?>
+--EXPECT--
+require_once(does-not-exist.php): Failed to open stream: No such file or directory

Zend/zend_execute.c

@@ -4224,7 +4224,7 @@ static zend_never_inline zend_op_array* ZEND_FASTCALL zend_include_or_eval(zval
 already_compiled:
 						new_op_array = ZEND_FAKE_OP_ARRAY;
 					}
-				} else {
+				} else if (!EG(exception)) {
 					zend_message_dispatcher(
 						(type == ZEND_INCLUDE_ONCE) ?
 							ZMSG_FAILED_INCLUDE_FOPEN : ZMSG_FAILED_REQUIRE_FOPEN,