Fix --rich-traceback-extra-lines cli flag

* Merge pull request #5028

redbot/core/cli.py

@@ -74,19 +74,26 @@ async def interactive_config(red, token_set, prefix_set, *, print_header=True):
     return token
 
 
-def positive_int(arg: str) -> int:
+def non_negative_int(arg: str) -> int:
     try:
         x = int(arg)
     except ValueError:
-        raise argparse.ArgumentTypeError("Message cache size has to be a number.")
+        raise argparse.ArgumentTypeError("The argument has to be a number.")
+    if x < 0:
+        raise argparse.ArgumentTypeError("The argument has to be a non-negative integer.")
+    if x > sys.maxsize:
+        raise argparse.ArgumentTypeError(
+            f"The argument has to be lower than or equal to {sys.maxsize}."
+        )
+    return x
+
+
+def message_cache_size_int(arg: str) -> int:
+    x = non_negative_int(arg)
     if x < 1000:
         raise argparse.ArgumentTypeError(
             "Message cache size has to be greater than or equal to 1000."
         )
-    if x > sys.maxsize:
-        raise argparse.ArgumentTypeError(
-            f"Message cache size has to be lower than or equal to {sys.maxsize}."
-        )
     return x
 
 
@@ -230,7 +237,7 @@ def parse_cli_flags(args):
     )
     parser.add_argument(
         "--message-cache-size",
-        type=positive_int,
+        type=message_cache_size_int,
         default=1000,
         help="Set the maximum number of messages to store in the internal message cache.",
     )
@@ -265,7 +272,7 @@ def parse_cli_flags(args):
     )
     parser.add_argument(
         "--rich-traceback-extra-lines",
-        type=positive_int,
+        type=non_negative_int,
         default=0,
         help="Set the number of additional lines of code before and after the executed line"
         " that should be shown in tracebacks generated by Rich.\n"