Produce a more descriptive error when failing to parse a pubspec.

sdk/lib/_internal/pub/lib/src/pubspec.dart

Index: sdk/lib/_internal/pub/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub/lib/src/pubspec.dart b/sdk/lib/_internal/pub/lib/src/pubspec.dart
index ac8e6cdba25e730ad6c855f89d9f014bfb23a40e..87ec55d92d84610c57783fbaad15a03179591ec2 100644
--- a/sdk/lib/_internal/pub/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub/lib/src/pubspec.dart
@@ -221,8 +221,12 @@ class Pubspec {
           'Could not find a file named "pubspec.yaml" in "$packageDir".');
     }
 
-    return new Pubspec.parse(readTextFile(pubspecPath), sources,
-        expectedName: expectedName, location: pubspecUri);
+    try {
+      return new Pubspec.parse(readTextFile(pubspecPath), sources,
+          expectedName: expectedName, location: pubspecUri);
+    } on YamlException catch (error) {
+      throw new PubspecException("Error parsing $pubspecPath: $error");
+    }
   }
 
   Pubspec(this._name, this._version, this._dependencies, this._devDependencies,