Yaml export errors now don't show stack trace

[apainintheneck]

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

This is a fix for the bug mentioned in #1227 related to failed YAML exports showing the stack trace along with an error message. Now it just shows the error message.

Instead of printing an error message and returning none, I changed it to throw a RuntimeError. The advantage of this is that the calling function doesn't throw a TypeError when it gets back a none value instead of a string. It also gives us the opportunity to clean up the file that was open.

Closes #1227.

[micahellison]

Looks great! Thanks for your contributions.