Roll back impl core::error::Error

[hsivonen]

Fixes #992.

[hsivonen]

Not sure if we should do this, but preparing the PR in case we decide to.

[codecov]

Codecov Report

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #994   +/-   ##
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  Coverage        ?   80.53%           
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  Files           ?       24           
  Lines           ?     4253           
  Branches        ?        0           
=======================================
  Hits            ?     3425           
  Misses          ?      828           
  Partials        ?        0           

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[hsivonen]

Need to verify that this actually makes things work with sqlx-core in the depedency graph on Rust < 1.81.

[hsivonen]

The failure for sqlx-core's url and idna to unify with the top level's interferes with testing this with a git override locally!

[hsivonen]

After looking at this more, I think it would be better to get sqlx to change. sqlx opted out of default features for url when there weren't any, which isn't cool.

[hsivonen]

Let's try to get sqlx fixed: launchbadge/sqlx#3589