Use ToString for error messages

[dupdob]

Is your feature request related to a problem? Please describe.
Despite all effort and progress of NFluent, sometimes the description of the sut is lacking in the error message.
NFluent could use the sut ToString override when it exists to improve error messages.

Describe the solution you'd like
NFluent should use ToString override when available, or it could revert ton this when internal NFluent logic fails to provide useful reports.

Additional context
See error #355 for context