hush: fix EXIT trap recursion case; check redirection failures

function                                             old     new   delta                          
run_pipe                                            1299    1328     +29                          
hush_exit                                             90     102     +12                          
hush_main                                           1172    1179      +7                          
run_list                                            1226    1225      -1                          
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(add/remove: 0/0 grow/shrink: 3/1 up/down: 48/-1)              Total: 47 bytes 

5 files changed