Fix several problems with offline mode.

lib/src/source/hosted.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.source.hosted;
 
 import 'dart:async';
 import 'dart:io' as io;
 import "dart:convert";
 
@@ skipping 234 matching lines @@
     var parsed = _parseDescription(description);
     var server = parsed.last;
     log.io("Finding versions of $name in "
         "$systemCacheRoot/${_urlToDirectory(server)}");
     var versions = await _getCachedPackagesInDirectory(_urlToDirectory(server))
         .where((package) => package.name == name)
         .map((package) => package.pubspec)
         .toList();
 
     // If there are no versions in the cache, report a clearer error.
-    if (versions.isEmpty) fail("Could not find package $name in cache.");
+    if (versions.isEmpty) {
+      throw new PackageNotFoundException(
+          "Could not find package $name in cache.");
+    }
 
     return versions;
   }
 
   Future<bool> _download(String server, String package, Version version,
       String destPath) {
     // Since HostedSource is cached, this will only be called for uncached
     // packages.
     throw new UnsupportedError("Cannot download packages when offline.");
   }
 
-  Future<Pubspec> doDescribeUncached(PackageId id) {
-    // [getVersions()] will only return packages that are already cached.
-    // [CachedSource] will only call [doDescribeUncached()] on a package after
-    // it has failed to find it in the cache, so this code should not be
-    // reached.
-    throw new UnsupportedError("Cannot describe packages when offline.");
+  Future<Pubspec> describeUncached(PackageId id) {
+    throw new PackageNotFoundException(
+        "${id.name} ${id.version} is not available in your system cache.");
   }
 }
 
 /// Given a URL, returns a "normalized" string to be used as a directory name
 /// for packages downloaded from the server at that URL.
 ///
 /// This normalization strips off the scheme (which is presumed to be HTTP or
 /// HTTPS) and *sort of* URL-encodes it. I say "sort of" because it does it
 /// incorrectly: it uses the character's *decimal* ASCII value instead of hex.
 ///
@@ skipping 80 matching lines @@
   var name = description["name"];
   if (name is! String) {
     throw new FormatException("The 'name' key must have a string value.");
   }
 
   var url = description["url"];
   if (url == null) url = HostedSource.defaultUrl;
 
   return new Pair<String, String>(name, url);
 }