show 'cargo-expand' errors when testing an exercise

[Yehuda-blip]

Hey, I started going through the repo, It seems this is gonna be a great way for me to learn about macros!

When I tried to run the testing for the first exercise, the diff output was empty. I tried to deliberately fail the exercise but the diff was still empty, and I got a success result printed. Some debugging later it seems I skipped the "install cargo-expand" instruction in the readme. This PR just checks that the expand command ran successfully before printing results.

Steps to reproduce the problem:

• uninstall cargo-expand
• test exercise 1
  The diff should be empty, both when expecting success and both when expecting failure.

[alexanderankin]

potentially related to #59 ?

[Yehuda-blip]

As far as I can tell, the issues are not related. In order to initially build the crate I followed the instructions in #59, (remove lock and build) - this solved the build issue. However, I did not have cargo-expand installed and this led to empty diffs with the run -- test command.