Bugfix: append traceback to existing output for errored runs

[LudovicoRighi]

Related note:

apsis/python/apsis/apsis.py

Lines 339 to 340 in e5f1029

	# FIXME: For now, use the name "output" as this is the only one
	# the UIs render. In the future, change to "traceback".

---

Tested this for:

runs that produce output

output261 bytes
some_output...
some_output...
some_output...
Traceback (most recent call last):
  File "/home/lrighi/apsis/python/apsis/running.py", line 101, in _process_updates
    raise FileNotFoundError("TTTSimulating an issue!")
FileNotFoundError: TTTSimulating an issue!

runs that don't produce any output

output216 bytes
Traceback (most recent call last):
  File "/home/lrighi/apsis/python/apsis/running.py", line 101, in _process_updates
    raise FileNotFoundError("TTTSimulating an issue!")
FileNotFoundError: TTTSimulating an issue!

UPDATE: need to review this again; we probably shouldn't mix output and Apsis errors.