simpleperf: Separate EOF and error in ReadFileFeature.

Currently ReadFilFeature() wrongly prints error message when
EOF (no more file feature). And callers don't know if there is
an error.

So don't print error when EOF, and add parameter to return error.

Bug: none
Test: run simpleperf_unit_test
Change-Id: Ica7c68971f87d70864836cdcba380eabe5c63a1a
7 files changed