Standardize Pyreemote.ReemoteWrapper exec result

[XaviTorello]

ReemoteWrapper.execute_request result must be improved to be able to integrate

• error flag
• message detail
• error message detail
• optional return code //review if it's needed

It applies to https://github.com/gisce/reemote/blob/adapt_code_to_allow_calls/pyreemote/pyreemote/telemeasure.py#L94

Proposed return schema:

{
    'error': True / False,
    'message': $stdout,
    'error_message': $sterr,
    'rc': $execution_return_code,
}