contrib/mbox-split: abort after usage if wrong number of args

Previously, attempting to run the script without args will print:

$ ./mbox-split
Usage: ./mbox-split <input.mbox> <max size with SI suffix>
Traceback (most recent call last):
  File "/tmp/./mbox-split", line 25, in <module>
    if sys.argv[2].endswith(suffix):
IndexError: list index out of range

contrib/mbox-split

@@ -10,6 +10,7 @@ import sys
 
 if len(sys.argv) != 3:
     print(f"Usage: {sys.argv[0]} <input.mbox> <max size with SI suffix>")
+    sys.exit(1)
 
 suffixes = {
     "KiB": pow(2, 10),