Deprecate Pry.config.exception_whitelist,
use Pry.config.unrescued_exceptions instead.
Deprecate Pry::DEFAULT_EXCEPTION_WHITELIST,
use Pry::DEFAULT_UNRESCUED_EXCEPTIONS instead.
What white / black means are not clear,
use clearer terminologies.
This kind of error message seems to be updated in Ruby 2.2.
For example, the following code throws an error that has different
error message with Ruby version:
```
"".freeze.instance_eval do
def hi
end
end
```
In Ruby 2.1.3:
> RuntimeError: can't modify frozen Class
In Ruby 2.2.0-preview1:
> RuntimeError: can't modify frozen object
This finishes up Robert Gleeson's refactoring. A sole file for
RescuableException doesn't make much sense, since we have a few more
exceptions left behind.
