Add a macro for `return Err(human(format!(...)))`

Add a macro for `return Err(human(format!(...)))`

This pattern showed up quite a few locations throughout the codebase but it
ended up meaning that there were some massive levels of indentation when you
actually get to the formatting string. This commit adds a new `bail!` macro
which shortens this idiom to help get the indentation under control throughout
the codebase.