A better error handling inside the isChunk(..) function

In case when the build failed due to an error during building of some
module, this change ensures that plugin throws the original error,
rather than creates a new Error with a non-specific message.

Author: birdofpreyru

index.js

@@ -356,7 +356,19 @@ function getPath(compilation, source, chunk) {
 
 function isChunk(chunk, error) {
 	if (!(chunk instanceof Chunk)) {
-		throw new Error(typeof error === 'string' ? error : 'chunk is not an instance of Chunk');
+    /* If the entry module failed to build, throw the error that caused it. */
+    var e = chunk && chunk.entryModule && chunk.entryModule.error;
+    if (e) throw e;
+
+    /* If any of dependency modules failed to build, throw the first found
+     * error that caused it. */
+    chunk._modules.forEach((m) => {
+      if (m.error) throw m.error;
+    });
+
+    /* Otherwise throw an error with the error message passed in, or with
+     * the fallback error message. */
+    throw new Error(typeof error === 'string' ? error : 'chunk is not an instance of Chunk');
 	}
 
 	return true;