Convert FleetAPI error response to OwnerAPI format

[RichieB2B]

The FleetAPI proxy does not fill reason:

{'result': False, 'string': 'car could not execute command: disconnected'}, 'error': '', 'error_description': ''}

This patch converts the string back to reason.

[MikeBishop]

Why not use the findReason() function defined in #585?

[RichieB2B]

Because I need to double check existing solutions before starting coding! ;-)