Fix error raised on NULL return

Empty values, such as those returned from functions run for their side
effects, like simple print statements that write to files, are already
handled correctly internally. The are stored as 0 bytes in voidstar
format. However, in the final checking in the nexus I was raising an
error if no content was returned, this led to an incorrect failing state
after successfully running the code. That is now fixed.

However, in the course of testing and fixing this code, I was reminded
of an old problem with morloc: how to handle STDERR and STDOUT. In the
past, I passed data between pools and the nexus through the STDOUT
channel. In order for this to work, I had to ensure that the channel was
not corrupted by any STDOUT written by the wrapped functions. So I
captured any STDOUT from these runs and stored it in the MorlocMonad for
possible later use. Same with STDERR. This behavior, though, hampers
debugging and is no longer necessary since I pass data over sockets.

data/nexus.py

@@ -448,44 +448,52 @@ def print_return(data, schema_str):
         clean_exit(1, f"Implementation bug: expected data packet: {str(data)}")
 
     content = None
-    isgood = True
-    error = ""
 
-    if cmd_source == PACKET_SOURCE_MESG:
-        content = data[data_start: ]
-    elif cmd_source == PACKET_SOURCE_FILE:
-        filename = data[data_start:].decode()
-        with open(filename, 'rb') as file:
-            content = file.read()
-        os.unlink(filename)  # Delete the temporary file
-    elif cmd_source == PACKET_SOURCE_RPTR:
-        relptr = _unpack("Q", data[data_start:])[0]
-        _log(f"Made relative pointer {relptr!s} for return")
-        content = mp.from_shm(relptr, schema_str)
-
-    if content == None:
-        error = "Unexpected source"
-        isgood = False
+    try:
+        if cmd_source == PACKET_SOURCE_MESG:
+            content = data[data_start: ]
+        elif cmd_source == PACKET_SOURCE_FILE:
+            filename = data[data_start:].decode()
+            with open(filename, 'rb') as file:
+                content = file.read()
+            os.unlink(filename)  # Delete the temporary file
+        elif cmd_source == PACKET_SOURCE_RPTR:
+            relptr = _unpack("Q", data[data_start:])[0]
+            _log(f"Made relative pointer {relptr!s} for return")
+            content = mp.from_shm(relptr, schema_str)
+    except Exception as e:
+        errmsg = f"Failed to open return data packet: {e!s}"
+        clean_exit(1, errmsg)
+
+    if content == None and schema_str == "z":
+        # if we expected a NULL result and got one, do nothing
+        pass
+    elif content == None and schema_str != "z":
+        errmsg = "Return data was expected but none was found"
+        clean_exit(1, errmsg)
     elif cmd_format == PACKET_FORMAT_JSON:
         print(content, file=outfile)
-        isgood = True
-
     elif cmd_format == PACKET_FORMAT_MSGPACK:
         try:
             json.dump(mp.mesgpack_to_py(content, schema_str), fp = sys.stdout, separators = JSON_SEPARATORS)
+            print() # newline
         except Exception as e:
-            _log(trace(f"Failed to read output MessagePack format with error {str(e)}:\n hex = {hex(content)}\n chr = {str(content)}"))
-        print() # newline
-        isgood = True
+            errmsg = f"Failed to read output MessagePack format with error:\n{e!s}"
+            _log(trace(errmsg))
+            clean_exit(1, errmsg)
     elif cmd_format == PACKET_FORMAT_VOIDSTAR:
-        json.dump(content, fp = sys.stdout, separators = JSON_SEPARATORS)
-        print() # newline
-
-    # Exit with the proper error code
-    if isgood:
-        clean_exit(exit_code)
+        try:
+            json.dump(content, fp = sys.stdout, separators = JSON_SEPARATORS)
+            print() # newline
+        except Exception as e:
+            errmsg = f"Failed to read output voidstar format with error:\n{e!s}"
+            _log(trace(errmsg))
+            clean_exit(1, errmsg)
     else:
-        clean_exit(1, error)
+        errmsg = "Unexpected source"
+        clean_exit(1, errmsg)
+
+    clean_exit(exit_code)
 
 
 def _make_header(