Engine error handling improvements

Added realtime error messages for VM errors that abort program
execution.

Realtime error reporting enabled by default. The A2_RTERRORS flag
has been removed.

Added A2_SILENT and A2_RTSILENT flags to disable error messages
from API and engine/realtime contexts respectively. (A2_SILENT
not yet implemented!)

On off-line states, a2r_Error() now bypasses the response FIFO
and prints directly to stderr.