More accurate, helpful error message for handlebars version errors.

Makes the error messages regarding Handlebars versions more explicit and correct. Here is a new message for when Handlebars is missing, and a second for when the `COMPILER_REVISION` is incorrect.

![](http://cl.ly/image/0m3J2H0p220e/Screen%20Shot%202013-05-01%20at%2012.36.26%20AM.png)
![](http://cl.ly/image/3m311g2m0T3I/Screen%20Shot%202013-05-01%20at%2012.35.10%20AM.png)

The existing error of `Ember Handlebars requires Handlebars 1.0.0-rc.3 or greater` has been inaccurate since https://github.com/wycats/handlebars.js/commit/baccdb4cfc241cbcfd02d21aada56100c6c9c132 landed.