BitException::getErrorCode() marked as 'const'

[DymOK93]

Description

BitException::getErrorCode() marked as const

Motivation and Context

Closes #63

How Has This Been Tested?

Build the following code:

// main.cpp
#include <bit7zlibrary.hpp>
#include <iostream>
#include <iomanip>

int main {
  try {
    bit7z::Bit7zLibrary lib;
  } catch (const BitException& exc) {
    std::cerr << "Unable to unpack an archive: error " << std::hex << exc.getErrorCode() << std::endl;
  }
  return 0;
}

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[DymOK93]

Sorry, I forgot that MSVC2012 doesn't support the noexcept specifier. Fixed with forced push.