[ENHANCEMENT] Unify messaging for non-existent and not supported devices.

[mku514k]

Description

As for now, if ledctl is run with unsupported device, it returns:

ledctl: /dev/urandom: device not supported
ledctl: IBPI LOCATE: missing block device(s)... pattern ignored.

...but for devices that do not exist, it just returns:

ledctl: IBPI LOCATE: missing block device(s)... pattern ignored.

For consistency, an error message about device not existing needs to be added.

Reason

Internal discussion.
Consistency in behavior.

Environment

-- not relevant --

[mku514k]

PR merged, closing.