replace unwrap() with expect(format!(...))

[durka]

unwrap() produces panics with no explanation, and on some platforms there are no line numbers in the backtrace even in debug builds (rust-lang/rust#24346). expect() with an informative message is a much better user experience.

before:

$ ag -Q 'unwrap(' src | wc -l
  15

after:

$ ag -Q 'unwrap(' src | wc -l
   0

I made my best guess for an explanation at each error location, but they should be audited by someone who knows how the code works!

[johannhof]

This looks useful in the short term, but most of these errors should be handled properly and converted to warnings instead.

For anything that is so critical that a crash is completely inevitable, we should create our own CobaltError and return that to the build function.

So +1 from me, but let's make an issue on revisiting expect/unwrap handling.

Thanks for the contribution ❤️