Allow to report errors to the module through a less intrusive mechanism

[tsimbalar]

fixes #48

• Introduces new extension methods under namespace SerilogWeb.Classic.Extensions, mainly to be used by SerilogWeb.CLassic.WebAPI for https://github.com/serilog-web/classic-webapi/issues/16 :
  • HttpContext.AddSerilogWebError(Exception exception) / HttpContextBase.AddSerilogWebError(Exception exception)
  • HttpContext.GetLastSerilogWebError() / HttpContextBase.GetLastSerilogWebError()
• bump referenced Serilog version to v2.7.1 (published 9 month ago) . (v2.8.0 is only a month old, and forcing unnecessary update cold be annoying to end user)
• bump package version to v5.0 . Not really a breaking change but may introduce subtle behavior different when combined with secondary packages (WebAPI one)