Do not ignore HtmlConfig syntax errors silently

[TjeuKayim]

HtmlConfig parsing error are silently ignored and the default values are used. This makes it hard for users to troubleshoot typos in their book.toml.

I tried this book.toml:

[output.html]
default-theme = "navy"
curly-quotes = "true"
# should have been: curly-quotes = true

I expected to see this happen: mdBook displays a warning or error message when building if the configuration contains a syntax error, like
[WARNING] (mdbook::book): Configuration value for 'output.html.curly-quotes' is invalid, expected a boolean but got string "true".
It applies the valid config nodes and sets the default-theme to navy, but uses the default value for curly-quotes.

Instead, this happened: The build succeeds without any warning, and ignores the valid config nodes under output.html. So the default-theme falls back to default.

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mdBook version b4bb442