Fix #82; improve error message for invalid input

For example:

    >> (Class.new(ActiveInteraction::Base) { def execute; end }).run!(nil)
    ArgumentError: options must be a hash
      from .../lib/active_interaction/base.rb:40:in `initialize'
      from .../lib/active_interaction/base.rb:122:in `new'
      from .../lib/active_interaction/base.rb:122:in `run'
      from .../lib/active_interaction/modules/core.rb:47:in `run!'