OpenOnuAdapter OnuImage download to adapter: wrong error indication and possibly adapter crash

Two aspects of this bug report.
1.) When downloading the mage to the adapter runs into timeout (default 10s) a misleading image state (reason) is given, eg:
DEVICEID IMAGESTATE.VERSION IMAGESTATE.DOWNLOADSTATE IMAGESTATE.REASON IMAGESTATE.IMAGESTATE
40443373-3ba9-4863-a605-c0fec3dd0e62 V1.2.2 DOWNLOAD_FAILED OMCI_TRANSFER_ERROR IMAGE_UNKNOWN
As the effect has nothing to do with any OMCI processing another error code should be given. As there is currently no fitting error code defined in the voltha protos it would be better to use the 'general' error:
ImageState_UNKNOWN_ERROR.
Moreover in this case it must also be ensured, that possibly internally stored file descriptor indications have to be removed again.
2.) In specifc error situations the used http client of the adapter may fail with error inidcations to download the file. In such a situation we observed an adapter crash due to incorrect evaluation of the error indications. Happened e.g. here:
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0xe3dd37]

goroutine 6476 [running]:
github.com/opencord/voltha-openonu-adapter-go/internal/pkg/onuadaptercore.(*fileDownloadManager).downloadFile(0xc000124180, 0x1370460, 0xc0006dab10, 0xc0006e4120, 0x56, 0x11d002d, 0x4, 0xc00069a1c0, 0xc, 0x0, ...)
/go/src/internal/pkg/onuadaptercore/file_download_manager.go:294 +0xa37
github.com/opencord/voltha-openonu-adapter-go/internal/pkg/onuadaptercore.(*fileDownloadManager).StartDownload(0xc000124180, 0x1370460, 0xc0006dab10, 0xc00069a1c0, 0xc, 0xc0006e4120, 0x56, 0xc0001c0f20, 0x0)
/go/src/internal/pkg/onuadaptercore/file_download_manager.go:117 +0x231
github.com/opencord/voltha-openonu-adapter-go/internal/pkg/onuadaptercore.(*OpenONUAC).Download_onu_image(0xc000480000, 0x1370460, 0xc0006dab10, 0xc0006e8190, 0x3, 0x4, 0xc0006e8190)
/go/src/internal/pkg/onuadaptercore/openonu.go:669 +0x102d
github.com/opencord/voltha-lib-go/v5/pkg/adapters/common.(*RequestHandlerProxy).Download_onu_image(0xc0001241c0, 0x1370460, 0xc0006dab10, 0xc0005b10e0, 0x3, 0x4, 0x0, 0x0, 0x0)
/go/src/vendor/github.com/opencord/voltha-lib-go/v5/pkg/adapters/common/request_handler.go:940 +0xb9
reflect.Value.call(0x11bd5a0, 0xc0001241c0, 0x3a13, 0x11d00f5, 0x4, 0xc0006dac30, 0x2, 0x2, 0xc0004a5cc8, 0x44f94c, ...)
/usr/local/go/src/reflect/value.go:476 +0x8e7
reflect.Value.Call(0x11bd5a0, 0xc0001241c0, 0x3a13, 0xc0006dac30, 0x2, 0x2, 0xc0001241c0, 0x3a13, 0xc0004a5d98)
/usr/local/go/src/reflect/value.go:337 +0xb9
github.com/opencord/voltha-lib-go/v5/pkg/kafka.CallFuncByName(0x1370460, 0xc0006dab10, 0x11bd5a0, 0xc0001241c0, 0xc000671dd0, 0x12, 0xc0004a5f70, 0x1, 0x1, 0x3, ...)
/go/src/vendor/github.com/opencord/voltha-lib-go/v5/pkg/kafka/kafka_inter_container_library.go:777 +0x35a
github.com/opencord/voltha-lib-go/v5/pkg/kafka.(*interContainerProxy).handleMessage(0xc0003e20b0, 0x13703f0, 0xc000046010, 0xc0006daa50, 0x11bd5a0, 0xc0001241c0)
/go/src/vendor/github.com/opencord/voltha-lib-go/v5/pkg/kafka/kafka_inter_container_library.go:904 +0x17c5
created by github.com/opencord/voltha-lib-go/v5/pkg/kafka.(*interContainerProxy).waitForMessages
/go/src/vendor/github.com/opencord/voltha-lib-go/v5/pkg/kafka/kafka_inter_container_library.go:983 +0x93
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0xe3dd37]