[C++] Changes in the flathash program (#5255)
[C++] Changes in the flathash program (#5255)

* Correct the usage in the flathash program

As it is possible to have -- before the occurrence of the first
input STRING.

* Exit with 1 in the flathash program when an error occurs

royalharsh/flatbuffers,google/flatbuffers,evolutional/flatbuffers,evolutional/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,stewartmiles/flatbuffers,royalharsh/flatbuffers,alexames/flatbuffers,stewartmiles/flatbuffers,evolutional/flatbuffers,royalharsh/flatbuffers,DavadDi/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,stewartmiles/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,pjulien/flatbuffers,pjulien/flatbuffers,alexames/flatbuffers,DavadDi/flatbuffers,pjulien/flatbuffers,royalharsh/flatbuffers,stewartmiles/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,alexames/flatbuffers,google/flatbuffers,evolutional/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,DavadDi/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,google/flatbuffers,alexames/flatbuffers,google/flatbuffers,royalharsh/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,evolutional/flatbuffers,google/flatbuffers,pjulien/flatbuffers,royalharsh/flatbuffers,DavadDi/flatbuffers,evolutional/flatbuffers,google/flatbuffers,stewartmiles/flatbuffers,pjulien/flatbuffers,stewartmiles/flatbuffers,royalharsh/flatbuffers,alexames/flatbuffers,evolutional/flatbuffers,google/flatbuffers,stewartmiles/flatbuffers,google/flatbuffers,pjulien/flatbuffers,google/flatbuffers,pjulien/flatbuffers,DavadDi/flatbuffers,royalharsh/flatbuffers,pjulien/flatbuffers,google/flatbuffers,google/flatbuffers,stewartmiles/flatbuffers,royalharsh/flatbuffers,stewartmiles/flatbuffers,evolutional/flatbuffers,alexames/flatbuffers,royalharsh/flatbuffers,stewartmiles/flatbuffers,alexames/flatbuffers,pjulien/flatbuffers,evolutional/flatbuffers,google/flatbuffers,alexames/flatbuffers