Downgrade match_wild_err_arm to pedantic and update help messages

[elichai]

Hi,
This fixes #3688 and downgrades match_wild_err_arm to pedantic.
There are a lot of different reasons in that issue, for me the biggest are:

1. Rust's errors aren't like Java's exceptions because they're type safe and in most cases the type of error can't change by itself.
2. Sometimes matching can be more ergonomic, and before the track_caller feature got introduced it was actually easier to track the panic location with explicit panic! than with expect.

Currently clippy is failing to build because of a breaking change in rust-lang/rust#69171 I tried fixing it but it is too complex for my little knowledge of clippy and rustc so I'll leave that to people who know what they're doing :)

Another thing, if rustc is breaking clippy a lot then maybe it's better to use something like miri does, where it's hard codes the latest tested rustc commit and they keep bumping it, that way when you develop locally it should work even if there was a breaking change (https://github.com/rust-lang/miri/blob/master/rustup-toolchain#L23-L29)

changelog: Downgrade match_wild_err_arm to pedantic

[flip1995]

Thanks! We currently looking in pinning a nightly.

@bors r+

[bors]

📌 Commit 9ff599f has been approved by flip1995

[bors]

🔒 Merge conflict

This pull request and the master branch diverged in a way that cannot be automatically merged. Please rebase on top of the latest master branch, and let the reviewer approve again.

How do I rebase?

Assuming self is your fork and upstream is this repository, you can resolve the conflict following these steps:

1. git checkout 2020-05-match_wild_err_arm (switch to your branch)
2. git fetch upstream master (retrieve the latest master)
3. git rebase upstream/master -p (rebase on top of it)
4. Follow the on-screen instruction to resolve conflicts (check git status if you got lost).
5. git push self 2020-05-match_wild_err_arm --force-with-lease (update this PR)

You may also read Git Rebasing to Resolve Conflicts by Drew Blessing for a short tutorial.

Please avoid the "Resolve conflicts" button on GitHub. It uses git merge instead of git rebase which makes the PR commit history more difficult to read.

Sometimes step 4 will complete without asking for resolution. This is usually due to difference between how Cargo.lock conflict is handled during merge and rebase. This is normal, and you should still perform step 5 to update this PR.

Error message

Auto-merging src/lintlist/mod.rs
Auto-merging clippy_lints/src/matches.rs
Auto-merging clippy_lints/src/lib.rs
CONFLICT (content): Merge conflict in clippy_lints/src/lib.rs
Automatic merge failed; fix conflicts and then commit the result.

[bors]

☔ The latest upstream changes (presumably #5582) made this pull request unmergeable. Please resolve the merge conflicts.

[elichai]

@flip1995 Thanks!
I rebased on master and also pushed a commit updating a stderr change that is unrelated.

[flip1995]

@bors r+

Thanks!

[bors]

📌 Commit 2db7f1a has been approved by flip1995

[bors]

⌛ Testing commit 2db7f1a with merge 20f09e1...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing 20f09e1 to master...