Address a number of open issues.

CHANGELOG.md

 # Changelog
 
+## 2.1.1
+- Reduce max concurrent connections to the same host to 6 when using `dart:io`.
+  That's the same limit that many browsers use.
+- Trying to load a resource from a non-existing package now gives a better
+  error message.
+
 ## 2.1.0
 - Make failing HTTP requests throw an `HttpException`.
 
 ## 2.0.2
 - Update README.md.
 
 ## 2.0.1
 - Fix type warnings.
 
 ## 2.0.0
 - Use configuration dependent imports to avoid having separate implementations
   for `dart:io` and `dart:html`.
 - Remove `browser_resource.dart`.
 
 ## 1.1.0
 
 - Added browser-compatible version as `browser_resource.dart` library.
   Only needed because configurable imports are not available yet.
 
 ## 1.0.0
 
 - Initial version