Clean up error messages in the previous CL.

TIL PLOG already logs the errno message, so it got logged twice
SignalCatcher::Output. Switch to PLOG for the corresponding messages in
palette_android.cc for brevity, and switch to LOG in the caller since the
errno message already has been logged by then.

Test: adb shell killall -QUIT system_server with a nearly full data
  partition; check logcat output.
Bug: 130025619

(cherry picked from commit 9b70ee0fdb1969a9a5cad2052b35c2fa04509bc2)

Change-Id: Iccc19e3594fe7e8c6ff2fbdec9203c49190b06d1