Switched editor to Ace for better error message display.

Replaced the CodeMirror code editor with Ace editor (http://ace.c9.io/). A few points to note:

* The font looks nicer.
* The syntax highlighter for Whiley now highlights a few more types of tokens, and is fairly easy to extend.
* Error messages cause markers to be displayed in the gutter. Hovering over the marker will show the error message.
* There is also the ability to add "warning" messages as well as error messages.
* There are plenty of other extensions that can be added to the code editor if needed.

Also, this pull request fixes a bug where instead of displaying an error message when things go wrong at the OS level (e.g. Java or missing files) it crashed instead of displaying an error.