Catch syntax errors in context and fail gracefully

[sethkinast]

Reviving #267 with un-bitrotted code.

This catches three cases where errors in context functions would bomb execution:

1. _get would try to execute functions from the context. If they fail, it return dust.log(), which returns undefined.
2. map() would try to execute a callback, but the callback could throw.
3. sections can be functions that are executed, but the functions could throw.

All of these now log an ERROR level, and call setError() on their chunk.

The easiest way to see the effect is to introduce a syntax error into one of the test cases. The dust.render() callback will never be invoked, which cripples many Node-based implementations that use Dust rendering, since they just spin forever.

I did introduce another change in this PR: if tests are expecting an error, they now pass if the expected value is contained within the error, instead of checking equality. This is because PhantomJS has different errors compared to a browser.

[sixlettervariables]

+1, I had wondered why my Dust templates sometimes hung (node v0.10).

[prashn64]

lgtm, one more set of eyes.

[rragan]

lgtm