Clean up the semantics of package descriptions.

lib/src/solver/version_solver.dart

 // Copyright (c) 2012, 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 pub.solver.version_solver;
 
 import 'dart:async';
 import "dart:convert";
 
 import 'package:pub_semver/pub_semver.dart';
@@ skipping 178 matching lines @@
     var ids;
     try {
       ids = await source.getVersions(package);
     } catch (error, stackTrace) {
       // If an error occurs, cache that too. We only want to do one request
       // for any given package, successful or not.
       var chain = new Chain.forTrace(stackTrace);
       log.solver("Could not get versions for $package:\n$error\n\n" +
           chain.terse.toString());
       _versionErrors[package] = new Pair(error, chain);
-      throw error;
+      rethrow;
     }
 
     // Sort by priority so we try preferred versions first.
     ids.sort((id1, id2) {
       // Reverse the IDs because we want the newest version at the front of the
       // list.
       return _type == SolveType.DOWNGRADE
           ? Version.antiprioritize(id2.version, id1.version)
           : Version.prioritize(id2.version, id1.version);
     });
@@ skipping 216 matching lines @@
 
   DependencyNotFoundException(String package, this._innerException,
       Iterable<Dependency> dependencies)
       : super(package, dependencies);
 
   /// The failure isn't because of the version of description of the package,
   /// it's the package itself that can't be found, so just show the name and no
   /// descriptive details.
   String _describeDependency(PackageDep dep) => "";
 }