Ajax: Protect against exceptions thrown synchronously by xhr.send

When xhr.send throws an exception synchronously, the onerror handler may have
been called already which, unchecked, makes the exception bubble up outside of
jQuery.ajax.

We now catch the exception pre-emptively and only rethrow if we know it hasn't
already been notified through the onerror handler.

Fixes #14683

test/unit/ajax.js

@@ -1544,6 +1544,36 @@ module( "ajax", {
 		});
 	}
 
+	ajaxTest( "#14683 - jQuery.ajax() - Exceptions thrown synchronously by xhr.send should be caught", 4, [
+		{
+			url: "data/params_html.php",
+			method: "POST",
+			data: {
+				toString: function() {
+					throw "Can't parse";
+				}
+			},
+			processData: false,
+			done: function( data ) {
+				ok( false, "done: " + data );
+			},
+			fail: function( jqXHR, status, error ) {
+				ok( true, "exception caught: " + error );
+				strictEqual( jqXHR.status, 0, "proper status code" );
+				strictEqual( status, "error", "proper status" );
+			}
+		},
+		{
+			url: "http://domain.org:80d",
+			done: function( data ) {
+				ok( false, "done: " + data );
+			},
+			fail: function( _, status, error ) {
+				ok( true, "fail: " + status + " - " + error );
+			}
+		}
+	]);
+
 //----------- jQuery.ajaxPrefilter()
 
 	ajaxTest( "jQuery.ajaxPrefilter() - abort", 1, {