Unify Exceptions for io and web

[lexaknyazev]

Resource.readAsBytes() and friends throw different exceptions depending on which platform is being used. Given that dart:io and dart:html are generally mutually exclusive, there's no way to safely type-test thrown exceptions using something like on FileSystemException catch (e).

This package could throw its own exceptions instead with fields filled from the corresponding platform-specific exception classes.

• io_io.dart throws one of:

  class FileSystemException implements IOException {
    final String message;
    final String path;
    final OSError osError;
  }
  
  class HttpException implements IOException {
    final String message;
    final Uri uri;
  }
  

• io_html.dart should check HTTP response code and throw from _httpGetBytes.

Common API could look like:

class ResourceException {
  final String message;
  final Uri uri;
  final Uri resolvedUri;
}