Remove dust.isDebug and remove dust.onErrors return value

This is step 1 of removing/deprecating a few debugging features.

step 2) dust v2.4.x
- Deprecate error throwing in dust.log, since it's overreaching its domain.
- Deprecate dust.onError to rename it to dust.throwError, since dust.onError implies it's a callback.

step 3) dust v2.5.x
- Remove error throwing in dust.log
- Rename dust.onError -> dust.throwError