Wrap source id with quotes in unresolved error

src/Bundle.js

@@ -241,7 +241,7 @@ export default class Bundle {
 					let isExternal = this.isExternal( externalId );
 
 					if ( !resolvedId && !isExternal ) {
-						if ( isRelative( source ) ) throw new Error( `Could not resolve ${source} from ${module.id}` );
+						if ( isRelative( source ) ) throw new Error( `Could not resolve '${source}' from ${module.id}` );
 
 						this.onwarn( `Treating '${source}' as external dependency` );
 						isExternal = true;

test/function/throws-not-found-module/_config.js

@@ -5,6 +5,6 @@ module.exports = {
 	solo: true,
 	description: 'throws error if module is not found',
 	error: function ( err ) {
-		assert.equal( err.message, 'Could not resolve ./mod from ' + path.resolve( __dirname, 'main.js' ) );
+		assert.equal( err.message, 'Could not resolve \'./mod\' from ' + path.resolve( __dirname, 'main.js' ) );
 	}
 };