Add error handling to env config handling

This adds several changes to how environment variables are handled to
more closely align with how configs are handled, and to fix an issue
with replacing entire tables. The changes are:

- Top-level tables like `MDBOOK_BOOK` now *replace* the contents of the
  `book` table instead of merging it. This adds consistency with how all
  the other environment objects work.
- Fixed allowing top-level replacement of `MDBOOK_BOOK` and
  `MDBOOK_OUTPUT`. This was inadvertently recently broken.
- Added ability to replace top-level `MDBOOK_RUST`. I don't recall why
  that wasn't included.
- Reject invalid keys like `MDBOOK_FOO`.
- Reject unknown keys, like `MDBOOK_BOOK='{"xyz": 123}'`
- Reject invalid types, like `MDBOOK_BOOK='{"title": 123}'`

CHANGELOG.md

@@ -9,7 +9,10 @@ This release also includes many new features described below.
 
 We have prepared a [0.5 Migration Guide](#05-migration-guide) to help existing authors switch from 0.4.
 
-The final 0.5.0 release does not contain any changes since [0.5.0-beta.2](#mdbook-050-beta2).
+The final 0.5.0 release only contains the following changes since [0.5.0-beta.2](#mdbook-050-beta2):
+
+- Added error handling to environment config handling. This checks that environment variables starting with `MDBOOK_` are correctly specified instead of silently ignoring. This also fixed being able to replace entire top-level tables like `MDBOOK_OUTPUT`.
+  [#2942](https://github.com/rust-lang/mdBook/pull/2942)
 
 ## 0.5 Migration Guide
 
@@ -56,6 +59,10 @@ The following is a summary of the changes that may require your attention when u
   [#2775](https://github.com/rust-lang/mdBook/pull/2775)
 - Removed the very old legacy config support. Warnings have been displayed in previous versions on how to migrate.
   [#2783](https://github.com/rust-lang/mdBook/pull/2783)
+- Top-level config values set from the environment like `MDBOOK_BOOK` now *replace* the contents of the top-level table instead of merging into it.
+  [#2942](https://github.com/rust-lang/mdBook/pull/2942)
+- Invalid environment variables are now rejected. Previously unknown keys like `MDBOOK_FOO` would be ignored, or keys or invalid values inside objects like the `[book]` table would be ignored.
+  [#2942](https://github.com/rust-lang/mdBook/pull/2942)
 
 ### Theme changes