fix: provide useful stacktrace on chunk loading failure

Error created from load/error event handler has no useful stack trace
whatsoever as it's an async event and stack has unwound already at that
point.

To have better (sync) stacktrace of what triggered loading of the
chunk, create error before stack has unwound and use it and its
stacktrace later, in case of an error.

This potentially has some processing overhead as browser needs to
create a stacktrace. It's probably not that big though but I haven't
done any real testing.

lib/web/JsonpMainTemplatePlugin.js

@@ -173,6 +173,8 @@ class JsonpMainTemplatePlugin {
 								"}"
 						  ])
 						: "",
+					"// create error before stack unwound to get useful stacktrace later",
+					"var error = new Error();",
 					"onScriptComplete = function (event) {",
 					Template.indent([
 						"// avoid mem leaks in IE.",
@@ -185,7 +187,7 @@ class JsonpMainTemplatePlugin {
 							Template.indent([
 								"var errorType = event && (event.type === 'load' ? 'missing' : event.type);",
 								"var realSrc = event && event.target && event.target.src;",
-								"var error = new Error('Loading chunk ' + chunkId + ' failed.\\n(' + errorType + ': ' + realSrc + ')');",
+								"error.message = 'Loading chunk ' + chunkId + ' failed.\\n(' + errorType + ': ' + realSrc + ')';",
 								"error.type = errorType;",
 								"error.request = realSrc;",
 								"chunk[1](error);"