Make fail() in main.py use exit code 2

This is in line with the recommendation from python#6003.
(It is called if there's something wrong with a filename.)

Author: Guido van Rossum

mypy/main.py

@@ -1056,4 +1056,4 @@ def parse_section(prefix: str, template: Options,
 
 def fail(msg: str) -> None:
     sys.stderr.write('%s\n' % msg)
-    sys.exit(1)
+    sys.exit(2)