Readable package name parsing errors

[magnalite]

Errors during parsing of package names is confusing, see #13. This PR adds extra info to the errors output so users can understand what went wrong more easily.

Users will now see this when they have an invalid scope

failed to parse manifest at path /../../wally.toml

Caused by:
    package scope Magnalite in invalid (scopes can only contain lowercase characters, digits and '-') for key `package.name` at line 2 column 8

I'm not very happy with this but I'm not sure how else to do it. Ideally we would output multiple layers of anyhow context but this isn't easy to do as the toml parsing is what consumes the anyhow error.

[LPGhatguy]

Great change! Minor typo?