add _bg_exc kwarg

sh.py

@@ -586,6 +586,8 @@ def __init__(self, cmd, call_args, stdin, stdout, stderr):
         if call_args["bg"]:
             should_wait = False
 
+        self._bg_exc = call_args["bg_exc"]
+
         # redirection
         if call_args["err_to_out"]:
             stderr = OProc.STDOUT
@@ -932,6 +934,9 @@ class Command(object):
         # ends
         "bg": False,
 
+        # automatically report exceptions for background commands
+        "bg_exc": True,
+
         "with": False, # prepend the command to every command after it
         "in": None,
         "out": None, # redirect STDOUT
@@ -1984,7 +1989,7 @@ def timeout_fn():
             # RunningCommand.wait() does), because we want the exception to be
             # re-raised in the future, if we DO call .wait()
             handle_exit_code = None
-            if not self.command._spawned_and_waited:
+            if not self.command._spawned_and_waited and self.command._bg_exc:
                 def fn(exit_code):
                     with process_assign_lock:
                         return self.command.handle_command_exit_code(exit_code)