Consider errors logged by a transformer equivalent to thrown errors.

pkg/barback/lib/src/base_transform.dart

Index: pkg/barback/lib/src/base_transform.dart
diff --git a/pkg/barback/lib/src/base_transform.dart b/pkg/barback/lib/src/base_transform.dart
index 64945ee72eaac2decbc065e04992a52302e038a1..7c9221ba52455e045242f09d321a67ade874ee7e 100644
--- a/pkg/barback/lib/src/base_transform.dart
+++ b/pkg/barback/lib/src/base_transform.dart
@@ -29,6 +29,11 @@ abstract class BaseTransform {
   /// This is exposed via [BaseTransformController].
   bool _consumePrimary = false;
 
+  /// Whether the transformer logged an error.
+  ///
+  /// This is exposed via [BaseTransformController].
+  bool _loggedError = false;
+
   /// The controller for the stream of log entries emitted by the transformer.
   ///
   /// This is exposed via [BaseTransformController].
@@ -67,6 +72,8 @@ abstract class BaseTransform {
 
   BaseTransform(this._node) {
     _logger = new TransformLogger((asset, level, message, span) {
+      if (level == LogLevel.ERROR) _loggedError = true;
+
       // If the log isn't already associated with an asset, use the primary.
       if (asset == null) asset = _node.primary.id;
       var entry = new LogEntry(_node.info, asset, level, message, span);
@@ -123,6 +130,9 @@ abstract class BaseTransformController {
   /// Whether the primary input should be consumed.
   bool get consumePrimary => transform._consumePrimary;
 
+  /// Whether the transform logged an error.
+  bool get loggedError => transform._loggedError;
+
   /// The stream of log entries emitted by the transformer during a run.
   Stream<LogEntry> get onLog => transform._onLogController.stream;