Fix endSnippet block getting removed for missing snippet

[mysticmind]

• Fix endSnippet block getting removed for a missing snippet with
  config InPlaceOverwrite
• Wrap ** Count not find snippet... in a fenced code block.

[mysticmind]

The problem is outlined below:
In markdown, the following is the content with InPlaceOverwrite:

<!-- snippet: sample_FindJsonUserByUsername -->
<!-- endSnippet -->

When I run mdsnippets tool or via msbuild for a missing snippet, it strips off <!-- endSnippet --> as below:

<!-- snippet: sample_FindJsonUserByUsername -->
** Could not find snippet 'sample_FindJsonUserByUsername' **

This PR fixes the same.

I couldn't figure out a way to add unit tests for this case, please provide some clues, I will try to add the same.

FYI, I am using MarkdownSnippets for Marten v4 docs which is currently being worked upon by me.

[SimonCropp]

can u add a test to verify this change

[mysticmind]

I couldn't figure out a way to add unit tests for this case, please provide some clues, I will try to add the same.

I had added the above in my comment, still checking to see how to add a unit test.

[mysticmind]

@SimonCropp I am closing this PR for now. I am able to reproduce the issue using the latest NuGet version 23.1.3. But not on the latest codebase on Git.

[mysticmind]

I was able to reproduce this in the latest code in repo, I am committing the test shortly for your review.

[mysticmind]

@SimonCropp the PR is ready for your review and next steps. Also it would be super helpful if you could release it to NuGet at the earliest once you confirm this PR is good. Thank you!

[SimonCropp]

thanks. deployed