If the user could not create a hard link (due to permissions or
different block devices) then czkawka lost the duplicate file.
```
$ mkdir hardlink
$ cd hardlink
$ echo a > a
$ cp a b
$ chown thetestuser:thetestuser a
$ ls -il .
25169246 -rw-rw-r-- 1 thetestuser thetestuser 2 Feb 7 11:18 a
25169641 -rw-rw-r-- 1 thomas thomas 2 Feb 7 11:18 b
$ cargo run --bin czkawka_cli dup --directories $(pwd) -m 1 -f
test101.result --delete-method HARD
[...]
-------------------------------WARNINGS--------------------------------
Failed to link /home/thomas/Development/czkawka/hardlink/b ->
/home/thomas/Development/czkawka/hardlink/a
---------------------------END OF WARNINGS-----------------------------
$ ls -il .
25169246 -rw-rw-r-- 1 thetestuser thetestuser 2 Feb 7 11:18 a
```
Now czkawka keeps all files and the warning provides more information
why czkawka can't replace the duplicate with a hard link.
```
$ cargo run --bin czkawka_cli dup --directories $(pwd) -m 1 -f
test101.result --delete-method HARD
-------------------------------WARNINGS--------------------------------
Failed to link /home/thomas/Development/czkawka/hardlink/b ->
/home/thomas/Development/czkawka/hardlink/a (Operation not permitted (os
error 1))
---------------------------END OF WARNINGS-----------------------------
[...]
25169246 -rw-rw-r-- 1 thetestuser thetestuser 2 Feb 7 11:18 a
25169641 -rw-rw-r-- 1 thomas thomas 2 Feb 7 11:18 b
```
