collect all test failures before failing

[guswynn]

I found that iterating on mdbook test is fairly slow as any failure in any file halts the process
This prints the error with error~, marks a failure, and then moves on

I tested that the return code is 0 on a passing book (i had a working local version of the cargo book) and that its non-0 on a non-passing book (dummy_book in tests is good for this)

[ehuss]

Thanks! I have kinda wanted this in the past, too. I made a small wording change, which looked like maybe a typo.