Use correct error message casing for `io::const_error`s

Error messages are supposed to start with lowercase letters, but a lot
of `io::const_error` messages did not. This fixes them to start with a
lowercase letter.
I did consider adding a const check for this to the macro, but some of
them start with proper nouns that make sense to uppercase them.

See https://doc.rust-lang.org/1.85.0/std/error/trait.Error.html

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