Improve autoreload for import-time errors.

We already had a special case for SyntaxErrors, but NameErrors and other
import-time errors could leave a file unwatched.

tornado/autoreload.py

@@ -71,6 +71,7 @@
 import os
 import pkgutil
 import sys
+import traceback
 import types
 import subprocess
 
@@ -275,7 +276,16 @@ def main():
         logging.info("Script exited with status %s", e.code)
     except Exception, e:
         logging.warning("Script exited with uncaught exception", exc_info=True)
+        # If an exception occurred at import time, the file with the error
+        # never made it into sys.modules and so we won't know to watch it.
+        # Just to make sure we've covered everything, walk the stack trace
+        # from the exception and watch every file.
+        for (filename, lineno, name, line) in traceback.extract_tb(sys.exc_info()[2]):
+            watch(filename)
         if isinstance(e, SyntaxError):
+            # SyntaxErrors are special:  their innermost stack frame is fake
+            # so extract_tb won't see it and we have to get the filename
+            # from the exception object.
             watch(e.filename)
     else:
         logging.info("Script exited normally")