Firmware export: report fail if unencrypted volume is read-only

[alex-nitrokey]

Expected behaviour

When exporting the firmware, a failed export should be produced if unencrypted volume is set to read-only (=export must fail)

Current behaviour

Successful export is declared, although export wasn't possible because of read-online state of unencrypted volume

[szszszsz]

Note: in https://github.com/Nitrokey/nitrokey-storage-firmware/issues/83#issuecomment-455564409 user could export the firmware, without setting the UV to R/W mode. The failures in export might have been wrongly assigned to the R/O mode of UV, instead of the double mounting issue. Source code review shows, that device does the writing via own procedure, not honouring at all set access flag for the host.