Custom CanyonError types

[TheRustifyer]

Refactor

Currently, we rely on the std library error types to propagate to the client our own errors. This is one example.

async fn find_by_pk_with<'a, R, I>(
                value: &'a dyn canyon_sql::core::QueryParameter<'a>,
                input: I
            ) -> Result<Option<#ty>, Box<(dyn std::error::Error + Send + Sync + 'a)>>
                where
                    I: canyon_sql::core::DbConnection + Send + 'a,
                    R: RowMapper
            {
                Err(
                    std::io::Error::new(
                        std::io::ErrorKind::Unsupported,
                        "You can't use the 'find_by_pk_with' associated function on a \
                        CanyonEntity that does not have a #[primary_key] annotation. \
                        If you need to perform an specific search, use the Querybuilder instead."
                    ).into_inner().unwrap()
                )
            }

• The snippet above belongs to the count macro

There's some smells there. But the most important ones w'd be to get rid out of the inlined str there (could be in a static str elsewhere), the use of an incorrect Err type, and the required conversion (on the .into_inner())

The idea is to start to create custom error variants under a enumerated type CanyonError, so any variant can hold any arbitrary message or information.

Furthermore, this w'd be an intermediate step to get rid out of the ... impl std::Err::Error + ... syntax on all of the return types, for something more concise and "static", let's say.