Prevent load errors from killing the jruby-rspec guard

After guard starts, if a class was changed in a way that caused an error to be raised on load, the error was raising up into the guard stack, which caused the jruby-rspec guard to get [fired / deactivated](https://github.com/guard/guard/blob/ee133a6b1ef8f9eb20243545b456c618440543cb/lib/guard/runner.rb#L102). Guard needed to be reloaded to continue testing.

    20:30:52 - ERROR - Guard::JRubyRSpec failed to achieve its <run_on_changes>, exception was:
    > [#] NameError: undefined local variable or method `crap' for #<Class:0x239e06d7>
    > [#] org/jruby/RubyKernel.java:227:in `method_missing'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/.bundle/jruby/1.8/gems/activerecord-3.2.12/lib/active_record/dynamic_matchers.rb:55:in `method_missing'
    > [#] ./app/models/address.rb:2:in `Address'
    > [#] ./app/models/address.rb:1:in `(root)'
    > [#] org/jruby/RubyKernel.java:1058:in `load'
    > [#] org/jruby/RubyMethod.java:133:in `call'
    > [#] ./app/models/address.rb:88:in `reload_paths'
    > [#] org/jruby/RubyArray.java:1615:in `each'
    > [#] /vagrant/git/guard-jruby-rspec/lib/guard/jruby-rspec.rb:73:in `reload_paths'
    > [#] /vagrant/git/guard-jruby-rspec/lib/guard/jruby-rspec.rb:54:in `run_on_changes'
    > [#] org/jruby/RubyKernel.java:2080:in `send'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:99:in `run_supervised_task'
    > [#] org/jruby/RubyKernel.java:1183:in `catch'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:97:in `run_supervised_task'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:144:in `run_first_task_found'
    > [#] org/jruby/RubyArray.java:1615:in `each'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:142:in `run_first_task_found'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:79:in `run_on_changes'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:175:in `scoped_guards'> [#] org/jruby/RubyArray.java:1615:in `each'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:173:in `scoped_guards'
    > [#] org/jruby/RubyKernel.java:1183:in `catch'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:172:in `scoped_guards'
    > [#] org/jruby/RubyArray.java:1615:in `each'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:170:in `scoped_guards'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard/runner.rb:72:in `run_on_changes'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard.rb:105:in `setup_listener'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard.rb:368:in `within_preserved_state'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard.rb:365:in `within_preserved_state'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/guard-1.8.0/lib/guard.rb:104:in `setup_listener'
    > [#] org/jruby/RubyProc.java:270:in `call'
    > [#] org/jruby/RubyProc.java:220:in `call'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/listen-1.0.2/lib/listen/listener.rb:234:in `on_change'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/listen-1.0.2/lib/listen/listener.rb:266:in `initialize_adapter'
    > [#] org/jruby/RubyProc.java:270:in `call'
    > [#] org/jruby/RubyProc.java:220:in `call'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/listen-1.0.2/lib/listen/adapters/polling.rb:48:in `poll_changed_directories'
    > [#] /home/vagrant/ws/rental_express/ROOT/rails/vendor/gems/listen-1.0.2/lib/listen/adapter.rb:284:in `start_poller'
    > [#] org/jruby/RubyProc.java:270:in `call'
    > [#] org/jruby/RubyProc.java:224:in `call'
    21:13:33 - INFO - Guard::JRubyRSpec has just been fired

This patch instead outputs the error and its stacktrace using `Guard::UI.error`, cancels the current test run, and continues listening for changes.