Validate against {{{ and }}}

[zepumph]

Over in phetsims/babel#22, I found a couple of spots where translators had accidentally included a third curly brace in their template variable. When @solaolateju and I went through the 4 usages of these, we found that they were clearly bugs. Does this seem like an easy and worth while validation step for rosetta 2.0? I know we have validation to ensure we keep the template variable intact, so perhaps this wouldn't be too challenging to add in on top of things.

Feel free to close if it isn't worth it.