ionice: make -t more tolerant

ionice: make -t more tolerant

* replace errx() with warnx() for unknown -c class

  The right place to check I/O scheduler features is in kernel. We should
  not try to be more smart than kernel.

* make the code ready (robust) for unknown sched.classes

* fix -t behavior

	old version:
	  $ ionice -c 4 -t bash
	  ionice: bad prio class 4

	new version:
	  $ ionice -c 4 -t bash

Reported-by: Voelker, Bernhard" <bernhard.voelker@siemens-enterprise.com>
Signed-off-by: Karel Zak <kzak@redhat.com>