Add suggestion when returning `Result<_, A>` when `Result<_, B>` is expected

[estebank]

When returning Result<_, A> but Result<_, B> is expected and A: Into<B>, provide a suggestion to return Ok(x?) instead:

error[E0308]: mismatched types
  --> src/bin/ruby-stacktrace.rs:86:20
   |
86 |             return version;
   |                    ^^^^^^^ expected struct `failure::Error`, found enum `ruby_stacktrace::address_finder::AddressFinderError`
   |
   = note: expected type `std::result::Result<_, failure::Error>`
              found type `std::result::Result<_, ruby_stacktrace::address_finder::AddressFinderError>`
   = help: because `ruby_stacktrace::address_finder::AddressFinderError` implements `failure::Error`, you can use `?` to coerce it:
   |
86 |             return Ok(version?);
   |                    ^^^^^^^^^^^^

Case taken from https://jvns.ca/blog/2018/01/13/rust-in-2018--way-easier-to-use/