[llvm-objdump] - Cleanup the error reporting.

The error reporting function are not consistent.

Before this change:

* They had inconsistent naming (e.g. 'error' vs 'report_error').
* Some of them reported the object name, others - dont.
* Some of them accepted the case when there was no error. (i.e. error code or Error had a success value).

This patch tries to cleanup it a bit.

It also renames report_error -> reportError, report_warning -> reportWarning
and removes a full stop from messages.

Differential revision: https://reviews.llvm.org/D66418

llvm-svn: 369515