[lib/ObjectYAML] - Improve and cleanup error reporting in ELFState<ELFT> class.

The aim of this patch is to refactor how we handle and report error.

I suggest to use the same approach we use in LLD: delayed error reporting.
For that I introduced 'HasError' flag which triggers when we report an error.
Now we do not exit instantly on any error. The benefits are:

1) There are no more 'exit(1)' calls in the library code.
2) Code was simplified significantly in a few places.
3) It is now possible to print multiple errors instead of only one.

Also, I changed the messages to be lower case and removed a full stop.

Differential revision: https://reviews.llvm.org/D67182

llvm-svn: 371380