Trying to create an existing file crashes program.

Steps to reproduce:
1. Launch Hive.
2. Select File|New.
3. Choose or type in the name of an existing file.

Actual behavior:
The application crashes with an error like the following:

Error in application
Java::OrgJrubyExceptions::RaiseException - org.jruby.exceptions.RaiseException: File exists - /Users/marnen/Hive bundle
/Users/marnen/Documents/hive/src/document/document_model.rb:48:in create_file': File exists - File exists - /Users/marnen/Hive bundle (Errno::EEXIST)

    from /Users/marnen/Documents/hive/src/document/document_model.rb:26:in `filename='

[etc.]

Desired behavior:
The application should either reject the selection or ask if we want to overwrite -- but it shouldn't crash! :)