Fixes #5574

Fixes #5628

Compare numbers instead of strings.

* health.vim: Include v:throwpoint in error message

* health/provider.vim: Check for ruby executable

* health/provider.vim: Combine subprocess stdout and stderr

* test: Updated CheckHealth test

People using [1] would experience an error, because their markdown syntax
doesn't define the markdownCodeBlock group.

[1]: https://github.com/plasticboy/vim-markdown

Closes #5569

We can now use help links like in normal help pages. The bars around them will
be concealed as well.

* Health: link string "SUCCESS" to ModeMsg

So far we linked "SUCCESS" to the Function highlight group.

The newly introduced healthHelp group links to Idenfifier as it does for links
in normal help pages.

Now the problem is that the Function group links again to Identifier as well, so
both, "SUCCESS" and help links, would use the same colors.

* Health: don't use Markdown code blocks

Every line indented by more than 4 spaces would lose its highlighting otherwise.

* Health: add helper health#help_to_link()

Never throw an error when provider/clipboard.vim is sourced for the first time.
Save the error instead and expose it via `provider#clipboard#Error()`, mimicking
provider/python.vim.