efinix: program reports success (returns 0) when programming fails

[AndrewD]

Efinix: programming errors still result in a success return code so there is no way for a script to detect errors from the return value.

This interface needs to be able to propagate errors (not return void): https://github.com/trabucayre/openFPGALoader/blob/a39524636bcc0846a799ed1fb874c891fc350c1b/src/efinix.hpp#L30