collect and propogate loader errors

[jbeeko]

Currently if a generator operates over invalid page data it is not able to express this in a nice way. For example if a post.md file is missing title in the top matter this is the result:

An unexpected error happend: Exception has been thrown by the target of an invocation.
... more useless logging which tells you nothing

To see this happen simply add a new file to the /posts director while in watch mode. The result is an immediate exception and termination of watch mode.

My first thought was to catch the invocation exception and indicate which page is failing during the load/generation phase.

That results in this:

The expression for /Users/joergbeekmann/Documents/Fornax/FornaxDemo/generators/post.fsx either couldn't be compiled or experianced a runtime
[11:34:23] '/Users/joergbeekmann/Documents/Fornax/FornaxDemo/_public/posts/post2.html' generation failed

Better, at least you know which page it is. But there is no way for the generator to indicate what is wrong with the markdown on the page. In this case a missing title.

This PR explores how to collect errors that occur during the load phase and then surface them as notifications in the console. That results in the following:

[10:03:28] Watch mode started. Press any key to exit.
[10:03:28 INF] Smooth! Suave listener started in 35.71ms with binding 127.0.0.1:8080
[10:03:44] Changes detected: /Users/joergbeekmann/Documents/FornexDemo/posts/post3.md
BAD FILE: /Users/joergbeekmann/Documents/FornexDemo/posts/post3.md
[10:03:44] '/Users/joergbeekmann/Documents/FornexDemo/_public/index.html' generated in 161ms
[10:03:44] '/Users/joergbeekmann/Documents/FornexDemo/_public/about.html' generated in 126ms

This PR is a proof of concept to go with issue #44.

[Krzysztof-Cieslak]

I like it, thanks!