Improve error when writing mdbook serve from nested folder

[lquenti]

Fixes #3000

I hope this is the correct place to fix, doing it directly in the serve execute function seemed weird as there its not trivial to me that it being a NotFound error implies that its about the SUMMARY.md.

[ehuss]

Thanks! Can you please add a test for this? I think it would probably go in tests/testsuite/build.rs. The test would probably look something like basic_build, but would just need to call current_dir() to some subdirectory, and include expect_failure(). There's documentation at https://github.com/rust-lang/mdBook/blob/master/tests/testsuite/README.md on how to update the expected output.