fs: btrfs: hide duplicate 'Cannot lookup file' error on 'load'

fs: btrfs: hide duplicate 'Cannot lookup file' error on 'load'

Running commands such as 'load mmc 2:1 $addr $path' when path does not
exists will print an error twice if the file does not exist, e.g.:
```
Cannot lookup file boot/boot.scr
Failed to load 'boot/boot.scr'
```
(where the first line is printed by btrfs and the second by common fs
code)

Historically other filesystems such as ext4 or fat have not been
printing a message here, so do the same here to avoid duplicate.

The other error messages in this function are also somewhat redundant,
but bring useful diagnostics if they happen somewhere, so have been left
as printf.

Note that if a user wants no message to be printed for optional file
loads, they have to check for file existence first with other commands
such as 'size'.

Signed-off-by: Dominique Martinet <dominique.martinet@atmark-techno.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>