simpleperf: print warning message when failed to read symbol table from elf file.

Change return type of read elf functions to ElfStatus, and print warning message
based on situation.
Add corresponding test.

Bug: 29193936
Change-Id: Ie449f8be34cb579962cb73f4be3df9e4997270ca
Test: run simpleperf_unit_test.
14 files changed