Fixed missing NULL return check when spawning voices

a2_VoiceSpawn() didn't check the return from a2_VoiceNew(), and as
a result, the nesting level check (or an OOM) would result in a
segfault.

Voice allocation OOMEMORY and VOICENEST errors now reported via
a2r_Error().

Also bumped A2_NESTLIMIT from 32 to 256 - because recursive voice
spawns are a totally valid programming method!

Fixes #70.