Avoid catch-all rescue and using exceptions for control-flow

The catch-all rescue in lib/devise_capturable/strategy.rb:63 was hiding
the helpful error message in the previous commit message:

    undefined method `capturable_server' for Devise:Module

Rescuing "Exception" seems like a dubious practice in general: helpful
messages might not get propagated when they should, and many Exceptions
that do not inherit from StandardError cannot be meaningfully handled
anyway.

There is only one example that relies on this behavior in the spec: when
saving a user fails (by raising Exception).

Instead of using this exception for control-flow, just check the result
of user.save (no bang) and call "fail!" right there if needed.  Avoid
the top-level rescue all together rather than weakening it to something
like StandardError because a Devise's prominent warden strategy also has
no top-level rescue.
Add pristine Rails engine "dummy" app generated by EN

I want to run integration tests, and to do so I need a test app to run them
against.  Rails's engine generates them with:

    rails plugin new devise_capturable --mountable

in the test/dummy directory.  Generate it there and copy it into this
project to ensure compatibility with Expectnation's (EN's) Rails version
(3.2.18).  Use the more descriptive directory name, "rails_app", as
Devise does.
scratch: Load rails_app during testing

Tests pass, but this needs lots of cleanup.  Some changes were made
temporarily that need to be backed out.

Also, I'm not sure this actually enables integration tests.  ABout to
find out...