Suppress logging to STDERR when swallowing class load errors

This removes a log message that was added about a month ago.  

One of our applications makes some heavy use of STI and we were seeing a few error messages being written to STDERR by ThinkingSphinx::Context#load_models even though the indices were being generated correctly.  I believe the problem stems from trying to force loading models while the application is currently in the middle of loading.

For instance, we have something roughly similar to:

```ruby
class A < ActiveRecord::Base
  define_index do
  ....
  end
end

define B < A
end

define C < B
end

define SomeObserver < ActiveRecord::Observer
  observes :b, :c
end
```

What I see is that first, SomeObserver is loaded by rails, which in turn tries to constantize 'B', which triggers loading of class A and the definition of A's index.  Defining the index triggers a call to #load_models which will eventually try to constantize 'C', which will error out with a NameError on 'B'.  My hypothesis is that this a cycle prevention mechanism in the class loader to avoid infinite loops.

Since these errors are otherwise being ignored, this just skips the logging.