Fail fast when user tries to generate reports with parallel workers

Author: ilevkivskyi

mypy/main.py

@@ -102,6 +102,8 @@ def main(
         options.native_parser = True
         if options.cache_dir == os.devnull:
             fail("error: cache must be enabled in parallel mode", stderr, options)
+        if options.report_dirs:
+            fail("error: reports are not supported in parallel mode yet", stderr, options)
 
     if options.allow_redefinition and not options.local_partial_types:
         fail(