Slightly reformatted the style of backtraces.

Opening a pull-request for this to get some feedback on it. The primary reason for this was because I personally feel that the message is a bit easier to read using this new format.

Example of the old format:

```text
An exception occurred running /tmp/test.rb
    undefined local variable or method `number' on Foo::Bar (Module) (NameError)

Backtrace:
  Kernel(Module)#number (method_missing) at kernel/delta/kernel.rb:81
                           Foo::Bar.test at /tmp/test.rb:4
                       Object#__script__ at /tmp/test.rb:9
        Rubinius::CodeLoader#load_script at kernel/delta/codeloader.rb:68
        Rubinius::CodeLoader.load_script at kernel/delta/codeloader.rb:119
                 Rubinius::Loader#script at kernel/loader.rb:645
                   Rubinius::Loader#main at kernel/loader.rb:846
```

Example of the new format:

```text
An exception occurred running /tmp/test.rb:

    undefined local variable or method `number' on Foo::Bar (Module) (NameError)

Backtrace:

  Kernel(Module)#number (method_missing) at kernel/delta/kernel.rb:81
                           Foo::Bar.test at /tmp/test.rb:4
                       Object#__script__ at /tmp/test.rb:9
        Rubinius::CodeLoader#load_script at kernel/delta/codeloader.rb:68
        Rubinius::CodeLoader.load_script at kernel/delta/codeloader.rb:119
                 Rubinius::Loader#script at kernel/loader.rb:645
                   Rubinius::Loader#main at kernel/loader.rb:846
```