introduce a proper Error type for various conversions

[ahl]

Fixes #459

Introduces a new mod in the code output:

pub mod error {
    #[doc = r" Error from a TryFrom or FromStr implementation."]
    pub struct ConversionError(std::borrow::Cow<'static, str>);
    impl std::error::Error for ConversionError {}
    impl std::fmt::Display for ConversionError {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
            std::fmt::Display::fmt(&self.0, f)
        }
    }
    impl std::fmt::Debug for ConversionError {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
            std::fmt::Debug::fmt(&self.0, f)
        }
    }
    impl From<&'static str> for ConversionError {
        fn from(value: &'static str) -> Self {
            Self(value.into())
        }
    }
    impl From<String> for ConversionError {
        fn from(value: String) -> Self {
            Self(value.into())
        }
    }
}