Always fail `rollup` with a rejected promise, not a thrown error.

src/rollup.js

@@ -38,7 +38,11 @@ export function rollup ( options ) {
 		return Promise.reject( new Error( 'The `transform`, `load`, `resolveId` and `resolveExternal` options are deprecated in favour of a unified plugin API. See https://github.com/rollup/rollup/wiki/Plugins for details' ) );
 	}
 
-	validateKeys( options, ALLOWED_KEYS );
+	const error = validateKeys( options, ALLOWED_KEYS );
+
+	if ( error ) {
+		return Promise.reject( error );
+	}
 
 	const bundle = new Bundle( options );
 

src/utils/validateKeys.js

@@ -3,9 +3,11 @@ import { keys } from './object.js';
 export default function validateKeys ( object, allowedKeys ) {
 	const actualKeys = keys( object );
 
-	actualKeys.forEach( key => {
+	for ( let key of actualKeys ) {
 		if ( allowedKeys.indexOf( key ) < 0 ) {
-			throw new Error( `Unexpected key '${ key }' found, expected one of: ${ allowedKeys.join( ', ' ) }` );
+			return new Error(
+				`Unexpected key '${ key }' found, expected one of: ${ allowedKeys.join( ', ' ) }`
+			);
 		}
-	});
+	}
 }

test/test.js

@@ -69,6 +69,14 @@ describe( 'rollup', function () {
 				assert.equal( 'You must supply options.entry to rollup', err.message );
 			});
 		});
+
+		it( 'fails with invalid keys', function () {
+			return rollup.rollup({ entry: 'x', plUgins: [] }).then( function () {
+				throw new Error( 'Missing expected error' );
+			}, function ( err ) {
+				assert.equal( 'Unexpected key \'plUgins\' found, expected one of: entry, dest, plugins, external, onwarn, indent, format, moduleName, sourceMap, intro, outro, banner, footer, globals, transform, load, resolveId, resolveExternal', err.message );
+			});
+		});
 	});
 
 	describe( 'bundle.write()', function () {