As mentioned in #409, some comments mentioned to use `?`
over calling `.unwrap()` and to use a struct
instead of two Strings. This PR leverages `?` and a new struct called
`WasmPackVersion` with a `local` and `latest` field. This PR also
uses `PBAR.warn(warning_msg)` instead of `println!(warning_msg)`
to print the warning message.
When checking for wasm32 target we did check if the sysroot
contained ".rustup". While this covered the most common cases it didn't
work when using Docker. So checking for "rustup" instead covers both
cases.
Charles Lanahan