fix!: prevent double printing of conversion errors

[TheLostLambda]

Most of this is just the commit description, but here's a picture of the sort of diff this change produces on my end: You can see that I used to get the same error message twice (all errors coming from DecodeError::Conversion have been doubled!), but now the second, duplicate source line is removed.

Okay, then just the commit description:

Fields named source are treated as magical by thiserror and, when using #[derive(Error)] are assumed to contain a sub-error. When a DecodeError::Conversion is then formatted by miette, the conversion error message contained in source is then printed twice: once by the #[error("{}", source)] tag on DecodeError::Conversion, then a second time, implicitly, because miette believes that DecodeError::Conversion contains an unprinted sub-error.

This change simply renames the source field of DecodeError::Conversion to error, side-stepping thiserrors special treatment of fields named source.

BREAKING CHANGE: renamed public field source to error