Concatenate validation errors with semicolons instead of newlines so they show up correctly in Studio

[TIM-659](https://edx-wiki.atlassian.net/browse/TIM-659): Studio validation errors concatenated with a space, not a comma/semicolon

@srpearce Could you please review this change?  Right now, the message looks like this:

![image](https://cloud.githubusercontent.com/assets/2948394/3363139/d5ac625c-fb10-11e3-8ca9-ee61e398d9cf.png)

This PR changes the space to a semicolon (before the second "Example 2").  Obviously not ideal, but (I think) a reasonable approach given that we're shoving the error message into Studio's UI.  I'm not sure of a way to make the error messages less verbose without making the code much more complex.