When a recipe wraps cli tool and the tool exits with a non-zero code,
just adds its own extra exit error message along with the messages
from the tool. Introduce the `[no-exit-message]` attribute to suppress
this additional message.
This commit adds a `Loader` type, which can be used to load multiple
source strings. This was done to support the work on modules, but
coincidentally enabled consolidating errors, since now `Config::run`
can take a `&Loader`, and in the event of an error, return and `Error`
that borrows from loaded strings. Multiple error types have been
consolidated, and a bunch of ad-hoc error printing was removed.
