Some generifying over error categories code.
Exception inheritance change.

 - cocaine::error_t is now inherited from std::system error
 - also left constructor without explicit specifying error code - for simple purposes.
Tiny refactoring.

 - exception superclass is renamed.
 - some code duplicates removed.
Error codes are explicitly enumerated now.
Locator exception hierarchy has been added.

 - new constructor overload for exception classed
 - an error code without additional description can be passed to the `error_t`, `indexed_error_t` and `generic_error_t`.
Refactoring over boost asserts.

 - boost assert macro has been moved into separate file (cocaine/detail/assert.hpp), cause it is version dependent.