[obj2yaml] - Rework tool's error reporting logic for ELF target.

[obj2yaml] - Rework tool's error reporting logic for ELF target.

ELF.h contains two getSymbol methods
which seems to be used only from obj2yaml.

One of these methods calls another, which in turn
contains untested error message which doesn't
provide enough information.

Problem is that after improving only just that message,
obj2yaml will not show it,
("Error reading file: yaml: Invalid data was
encountered while parsing the file" message will be shown instead),
because internal errors handling of tool is based on ErrorOr<> class which
stores a error code and as a result can only show a predefined error string, what
actually isn't very useful.

In this patch, I rework obj2yaml's error reporting system
for ELF targets to use Error  Expected<> classes.
Also, I improve the error message produced
by getSymbol for demonstration of the new functionality.

Differential revision: https://reviews.llvm.org/D64631

llvm-svn: 366052