Add explicit check for generated xmldoc on startup

[Rast1234]

Currently app starts but returns 500 if XML docs are not generated. Add check for this file and throw exception with sane description to help users configure their project (should add GenerateDocumentationFile to csproj)

Tochka.JsonRpc/src/Tochka.JsonRpc.Swagger/Extensions.cs

Line 65 in f639891

options.IncludeXmlComments(xmlPath);