This commit removes the internal `Error` type in favor of exclusively
using the `failure::Error` type, simplifying error construction in a
number of locations and reducing the number of error types in play.
The standard library version of an `RwLock` may get poisoned if a thread
panics that holds a write lock. The `RwLock` from parking_lot [1] does
not get poisoned it instead released the lock on a panic. This allows us
to simplify the `ProgressOutput` API since it no longer returns any
errors. No more panics can occur on `ProgressOutput::drop()`. The
`Error` enum can thus be simplified as well because there is no need to
convert `PoisonErrors` anymore.
[1] https://github.com/Amanieu/parking_lot
Alex Crichton