devel/p5-failures: Add p5-failures 0.004

This module lets you define an exception hierarchy quickly and simply.

Here were my design goals:
- minimalist interface
- 80% of features in 20% of lines of code
- depend only on core modules (nearly achieved)
- support hierarchical error types
- identify errors types by name (class) not by parsing strings
- leave (possibly expensive) trace decisions to the thrower

Currently, failures is implemented in under 70 lines of code.

Failure objects are implemented with Class::Tiny to allow easy subclassing (see
custom::failures), but Class::Tiny only requires core modules, so other than
that exception, the 'core only' goal is achieved.