ENH: improve abort/exception handling in error class (#1780)

- previously setting FOAM_ABORT would preempt checks for throwing
  exceptions.

  Now check for throwing first, to allow try/catch code to do its job.
  However, ignore exception throwing for abort(). These are used
  infrequently in the code, but indicate that recovery is deemed
  impossible.

STYLE: use unique_ptr for internal stream buffer management