fs_mount for FAT FS does not distingush between no file system and other errors

[de-nordic]

Describe the bug
The fs_mount is supposed, when option enabled, to create new file system on a device if mount fails due to non-existing file system.

To Reproduce
Steps to reproduce the behavior:

1. modify test/subsys/fs/fat_fs_api/prj_native_posix.conf by replacing
   CONFIG_DISK_FLASH_DEV_NAME="flash_ctrl"
   with
   CONFIG_DISK_FLASH_DEV_NAME="flash_ctrl_bla"
2. west build -b native_posix --build-dir mkfs_err zephyr/tests/subsys/fs/fat_fs_api/ -- -DCONF_FILE=prj_native_posix.conf
3. mkfs_err/zephyr/zephyr.elf
4. Observe multiple
   E: fs mount error (-5)
   as the fs_mount tries to mount the file system and then create file system on non-existent device.

Expected behavior
Distinguish when there is device error and when there is no-file system error; exist when there is device error and attempt to mkfs when there is no file system.

Environment (please complete the following information):

• OS: Linux
• Toolchain Zephyr sdk 0.12
• Commit SHA Zephyr: 554101e

[de-nordic]

Configuration problem. Not an issue.