Make the 'stdout/stderr/exit code' available during exceptions

An error is raised when a ssh_execute command returns an exit code is
greater than 0. This change allows use to capture that information in
a rescue call.

Example:
    ```
    begin
      # ... do something
    rescue Veewee::Provider::Core::Helper::SshResult => result
      puts result.stdout
    end
    ```
Merge pull request #539 from aw/raise_data_on_ssh_error

Make the 'stdout/stderr/exit code' available during exceptions