Warn on missing closing }}

[arv]

Given the following:

<template bind>
  {{ xyz
</template>

We should console.warn here.

[eseidel]

<template if='xyz}}'></template> would also be nice to warn on.
Or hell, even <template if='xyz'>. Like compilers warn when you make a syntax error or your if clause evaluates to a constant, seems fitting for Polymer's template compiler to too.