node_io: Convert Node-style errors into Dart-style errors

[nex3]

One of the major API differences currently between node_io and dart:io is the errors: where dart:io throws various subclasses of IOException, node_io just throws Node.js's raw exceptions. This makes it difficult to use node_io to write error-safe code that works with both Dart and Node. It would be great if these exceptions were caught and wrapped in Dart-style exception classes to increase compatibility.