Return ErrorOr from getSection.

This also improves the logic of what is an error:

* getSection(uint_32): only return an error if the index is out of bounds. The
  index 0 corresponds to a perfectly valid entry.
* getSection(Elf_Sym): Returns null for symbols that normally don't have
  sections and error for out of bound indexes.

In many places this just moves the report_fatal_error up the stack, but those
can then be fixed in smaller patches.

llvm-svn: 241156

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