feat: Improve errors from try_deserialize

[barskern]

Enables user code to handle missing and invalid fields after calling try_deserialize instead of having to match and search in the string returned in ConfigError::Message.

Personally I deem the String::leak call safe for my usage, as I would expect to terminate my program on configuration errors anyhow, however we could certainly also change the expected field to be a Cow<str> or similar to handle both &'static str and String elegantly.

Part of #532

[coveralls]

Pull Request Test Coverage Report for Build 17951967632

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 17 of 23 (73.91%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.3%) to 65.182%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/error.rs	17	23	73.91%

Files with Coverage Reduction	New Missed Lines	%
src/error.rs	4	73.58%

Totals
Change from base Build 17860294716:	0.3%
Covered Lines:	966
Relevant Lines:	1482

---

💛 - Coveralls

[epage]

Looks like some more test cases need updating

[barskern]

Looks like some more test cases need updating

Yeah, I didn't run with --all-features. While going through it I also changed some of the places were we can now rather match on the enum than search in the string, to show that it should return a proper variant.

[epage]

Thanks!