Log deserialization errors for [html.config]

[TjeuKayim]

Fixes #1032

Log deserialization errors in Config::html_config(), instead of ignoring them.
After implementing this, the log appeared three times, caused by multiple calls to html_config(). Therefore, I de-duplicated that code.

Example

Using this book.toml:

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

The build log displays the error:

[~/src/mdBook]% cargo run -- build ./book-example
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `target/debug/mdbook build ./book-example`
2019-09-22 21:19:14 [INFO] (mdbook::book): Book building has started
2019-09-22 21:19:14 [INFO] (mdbook::book): Running the html backend
2019-09-22 21:19:14 [ERROR] (mdbook::utils): Error: Parsing configuration [output.html]
2019-09-22 21:19:14 [ERROR] (mdbook::utils):    Caused By: Couldn't deserialize the value
2019-09-22 21:19:14 [ERROR] (mdbook::utils):    Caused By: invalid type: string "true", expected a boolean for key `curly-quotes`

[ehuss]

Thanks!