luks2: Propagate error when reading area key fails

luks2: Propagate error when reading area key fails

When decrypting a given keyslot, all error cases except for one set up
an error and return the error code. The only exception is when we try to
read the area key: instead of setting up an error message, we directly
print it via grub_dprintf().

Convert the outlier to use grub_error() to allow more uniform handling
of errors.

Signed-off-by: Patrick Steinhardt <ps@kps.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>