Require using skip or error for follow-imports in dmypy

Author: msullivan

mypy/dmypy_server.py

@@ -97,6 +97,10 @@ def process_start_options(flags: List[str]) -> Options:
         sys.exit("dmypy: start/restart should not specify quick_and_dirty mode")
     if options.use_fine_grained_cache and not options.fine_grained_incremental:
         sys.exit("dmypy: fine-grained cache can only be used in experimental mode")
+    # Our file change tracking can't yet handle changes to files that aren't
+    # specified in the sources list.
+    if options.follow_imports not in ('skip', 'error'):
+        sys.exit("dmypy: follow-imports must be 'skip' or 'error'")
     return options