Fix reporting libgphoto2 errors via KIO

KIO::ERR_COULD_NOT_READ expects the error message parameter to be
the file that could not be read. Instead an error message from
libgphoto2 was given. Using KIO::ERR_SLAVE_DEFINED instead allows to
report the libgphoto error as part of the error message.

Example from before: Could not read file Bad parameters.
Example from after: Could not read. Reason: Bad parameters

REVIEW: 117745