use resolvers/loaders in order, dont catch errors

src/Bundle.js

@@ -1,6 +1,6 @@
 import Promise from 'es6-promise/lib/es6-promise/promise';
 import MagicString from 'magic-string';
-import attempt from './utils/attempt.js';
+import first from './utils/first.js';
 import { blank, keys } from './utils/object';
 import Module from './Module';
 import ExternalModule from './ExternalModule';
@@ -17,16 +17,12 @@ export default class Bundle {
 		this.entry = options.entry;
 		this.entryModule = null;
 
-		this.resolveId = ensureArray( options.resolveId )
+		this.resolveId = first( ensureArray( options.resolveId ).concat( defaultResolver ) );
-			.reduce( attempt, defaultResolver );
+		this.load = first( ensureArray( options.load ).concat( defaultLoader ) );
-
-		this.load = ensureArray( options.load )
-			.reduce( attempt, defaultLoader );
 
 		this.resolveOptions = {
 			external: ensureArray( options.external ),
-			resolveExternal: ensureArray( options.resolveExternal )
+			resolveExternal: first( ensureArray( options.resolveExternal ).concat( defaultExternalResolver ) )
-				.reduce( attempt, defaultExternalResolver )
 		};
 
 		this.loadOptions = {

src/utils/attempt.js

@@ -1,11 +0,0 @@
-// Given a `fallback` and a `preferred` function,
-// return a function that attempts to call the `preferred`.
-// If it fails, or returns undefined, call the fallback and return its value.
-export default function attempt ( fallback, preferred ) {
-	return function ( ...args ) {
-		const boundFallback = () => fallback( ...args );
-
-		return Promise.resolve( preferred( ...args ) )
-			.then( res => res === undefined ? boundFallback() : res, boundFallback );
-	};
-}

src/utils/first.js

@@ -0,0 +1,11 @@
+// Return the first non-falsy result from an array of
+// maybe-sync, maybe-promise-returning functions
+export default function first ( candidates ) {
+	return function ( ...args ) {
+		return candidates.reduce( ( promise, candidate ) => {
+			return promise.then( result => result != null ?
+				result :
+				Promise.resolve( candidate( ...args ) ) );
+		}, Promise.resolve() );
+	}
+}