[llvm-readobj/llvm-readelf] - Improve/cleanup the error reporting API.

urrently we have the following functions for error reporting:

--
LLVM_ATTRIBUTE_NORETURN void reportError(Twine Msg);
void reportError(Error Err, StringRef Input); 
void reportWarning(Twine Msg);
void reportWarning(StringRef Input, Error Err);
void warn(llvm::Error Err);
void error(std::error_code EC);
---

Problems are: naming is inconsistent, arguments order is inconsistent,
some of the functions looks excessive.

After applying this patch we have:

---
LLVM_ATTRIBUTE_NORETURN void reportError(Error Err, StringRef Input); 
LLVM_ATTRIBUTE_NORETURN void reportError(std::error_code EC, StringRef Input);
void reportWarning(Error Err, StringRef Input);
---

I'd be happy to remove reportError(std::error_code EC, StringRef Input) too, but it
is used by COFF heavily.

Test cases were updated, they show an improvement introduced.

Differential revision: https://reviews.llvm.org/D66286

llvm-svn: 369190