Catch YAML::SyntaxError, not just SyntaxError.

In 1.x versions of Psych, Psych::SyntaxError was a subclass of ::SyntaxError, so catching ::SyntaxError worked just fine.

In 2.x versions of Psych, Psych::SyntaxError is no longer a subclass of ::SyntaxError (it's a subclass of just ::Exception), so the exception was not getting caught.

We now just catch YAML::SyntaxError (YAML == Psych), not just ::SyntaxError.

@steverice OR @akavi