Auto merge of #5622 - elichai:2020-05-match_wild_err_arm, r=flip1995

Downgrade `match_wild_err_arm` to pedantic and update help messages

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 https://github.com/rust-lang/rust/pull/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

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