Must not abort if exception thrown in threadpool

[springmeyer]

There are many functions being used in the threadpool that might throw. Whenever this is the case there MUST be a try/catch around the async code. Otherwise a C++ exception will go unhandled and lead to an abort (aka terminate) which will stop the entire node process similar to a crash. This should never happen and bugs like this need to be avoided. Another case of this try/catch going missing is mapbox/carmen-cache#56.

I'm creating this issue because I just hit a case where glyphs.SerializeAsString() threw and caused an abort inside RangeAsync.

[springmeyer]

Handled by #128