Added more explanatory exceptions for bootstrapper locator and web.config section

This PR does the following:
 - [x] Added exceptions to `NancyHttpRequestHandler` for four scenarios
  - The bootstrapper specified in web.config is not found
  - The bootstrapper specified in web.config does not have a default ctor, or the default ctor throws.
  - The `type` attribute of the bootstrapper element in web.config is empty
  - The `assembly` attribute of the bootstrapper element in web.config is empty
 - [x] Added exceptions to `NancyBootstrapperLocatorFixture` for two scenarios
  - Multiple bootstrapper types are found (since we don't know which to pick, we might as well force the user to do it).
  - The located bootstrapper does not have a default ctor, or the default ctor throws.

I reused the `BootstrapperException` for all scenarios.

This should provide users with better, more explanatory messages when bootstrapper location/initialization fails :sparkles: