set errors on the chunk when catching errors so that the proper callbacks are always invoked when rendering or streaming
rename dust.onError to dust.throwError, stop returning a chunk from the same function, remove error throwing from dust.log, and set a chunk error when dust helper errors are caught
remove dust.throwError/dust.onError
remove dust.silenceErrors
Merge pull request #471 from prashn64/stream-errors

Errors thrown from render will now call populate the error in the callback. Previously, thrown errors will immediately fail and not call the callback.  Issue #381, 

Address #468: Errors thrown from stream now will invoke the `error` listener with the error object. Previously, these errors could not be caught and the process will hang.

Deprecated/remove: `dust.onError`, `dust.silenceErrors`. To see runtime errors, look at the error in the callback. 
breaking change: `dust.log` with an error no longer throws that error