Simplifying the 'Esc' message

The current message is:
Close this tool. Or press 'Esc' to exit

It has several problems:
* No full stop.
* Very long.
* Strange grammar: beginning a sentence with "Or" is unusual.

I simply put the Esc in parentheses, which is a common practice
with keyboard shortcuts.

Change-Id: Ib79ba8020885200ae2aaf7830ab337936c436752