[Object/ELF.h] - Improve error reporting.

The errors coming from ELF.h are usually not very
useful because they are uninformative. This patch is a
first step to improve the situation.

I tested this patch with a run of check-llvm and found
that few messages are untested. In this patch, I did not
add more tests but marked all such cases with a "TODO" comment.

For all tested messages I extended the error text to
provide more details (see test cases changed).

Differential revision: https://reviews.llvm.org/D64014

llvm-svn: 365183