Improve the documentation for [TransformLogger.error].

pkg/barback/lib/src/transform_logger.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library barback.transform_logger;
 
 import 'package:source_maps/span.dart';
 
 import 'asset_id.dart';
 import 'log.dart';
@@ skipping 38 matching lines @@
   void warning(String message, {AssetId asset, Span span}) {
     _logFunction(asset, LogLevel.WARNING, message, span);
   }
 
   /// Logs an error message.
   ///
   /// If [asset] is provided, the log entry is associated with that asset.
   /// Otherwise it's associated with the primary input of [transformer].
   /// If present, [span] indicates the location in the input asset that caused
   /// the error.
-  // TODO(sigmund,nweiz): clarify when an error should be logged or thrown.
+  ///
+  /// Logging any errors will cause Barback to consider the transformation to
+  /// have failed, much like throwing an exception. This means that neither the
+  /// primary input nor any outputs emitted by the transformer will be passed on
+  /// to the following phase, and the build will be reported as having failed.
+  ///
+  /// Unlike throwing an exception, this doesn't cause a transformer to stop
+  /// running. This makes it useful in cases where a single input may have
+  /// multiple errors that the user wants to know about.
   void error(String message, {AssetId asset, Span span}) {
     _logFunction(asset, LogLevel.ERROR, message, span);
   }
 }