proto: Remove error message which hides earlier error.

When chunked encoding fails, the following error is added to
the handle:

  [filename]: error in chunked encoding

However all paths returning -1 from receive_file_data already set a
more specific error, which this error hides.
proto: Remove another error message which hides earlier error.

When receive_file_data calls guestfs___recv_from_daemon, and that
function returns -1, it adds the following error to the handle:

  receive_file_data: parse error in reply callback

However this hides a more specific error message which was already
added by guestfs___recv_from_daemon.
daemon: mkfs: Use -I option to force mkfs.fat to write a filesystem over a whole device (RHBZ#1039995).

If you use virt-make-fs to create a partitionless FAT-formatted disk
image then currently you will get an error:

$ virt-make-fs --type=fat . /tmp/test.img
'mkfs' (create filesystem) operation failed.
Instead of 'fat', try 'vfat' (long filenames) or 'msdos' (short filenames).
mkfs: fat: /dev/sda: mkfs.fat: Device partition expected, not making filesystem on entire device '/dev/sda' (use -I to override) at /usr/bin/virt-make-fs line 508, <PIPE> line 1.
      ...propagated at /usr/bin/virt-make-fs line 518, <PIPE> line 1.

With this patch, the error goes away and partitionless disks can be
created.