Writes source at the end of the Display message

[brooksprumo]

Fixes #51

[brooksprumo]

Note that clippy fmt splits these into multiple lines. If instead we inline the variables into the format strings, the code can remain compact.

            // assume above there is a `let source = &self.source;`
            OpenFile => write!(formatter, "failed to open file `{path}`: {source}"),
            CreateFile => write!(formatter, "failed to create file `{path}`: {source}"),
            CreateDir => write!(formatter, "failed to create directory `{path}`: {source}"),
            SyncFile => write!(formatter, "failed to sync file `{path}`: {source}"),
            // etc

(Note, the code doesn't remain a single line for all the variants. Refer to this branch for the impl: https://github.com/brooksprumo/fs-err/blob/source-inline/src/errors.rs)