ec2_metadata: specify path in exception

The current message is rather unhelpful to establish what exactly fails,
eg:

    DEBUG: Plugin Ec2 threw #<RuntimeError: Encountered error retrieving EC2 metadata (returned 404 response)>
    DEBUG: /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/ohai-7.0.2/lib/ohai/mixin/ec2_metadata.rb:104:in `metadata_get'
    DEBUG: /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/ohai-7.0.2/lib/ohai/mixin/ec2_metadata.rb:120:in `block in fetch_metadata'
    DEBUG: /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/ohai-7.0.2/lib/ohai/mixin/ec2_metadata.rb:113:in `each'
    DEBUG: /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/ohai-7.0.2/lib/ohai/mixin/ec2_metadata.rb:113:in `fetch_metadata'
    DEBUG: /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/ohai-7.0.2/lib/ohai/mixin/ec2_metadata.rb:130:in `block in fetch_metadata'
    DEBUG: /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/ohai-7.0.2/lib/ohai/mixin/ec2_metadata.rb:113:in `each'
    DEBUG: /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/ohai-7.0.2/lib/ohai/mixin/ec2_metadata.rb:113:in `fetch_metadata'
    DEBUG: /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/ohai-7.0.2/lib/ohai/plugins/ec2.rb:52:in `block (2 levels) in <main>'