Detect old Closure directory and prompt to rename.

build.py

@@ -406,7 +406,13 @@ if __name__ == '__main__':
     calcdeps = import_path(os.path.join(os.path.pardir,
           'closure-library', 'closure', 'bin', 'calcdeps.py'))
   except ImportError:
-    print("""Error: Closure not found.  Read this:
+    if os.path.isdir(os.path.join(os.path.pardir, 'closure-library-read-only')):
+      # Dir got renamed when Closure moved from Google Code to GitHub in 2014.
+      print("Error: Closure directory needs to be renamed from"
+            "'closure-library-read-only' to 'closure-library'.\n"
+            "Please rename this directory.")
+    else:
+      print("""Error: Closure not found.  Read this:
 https://developers.google.com/blockly/hacking/closure""")
     sys.exit(1)
   search_paths = calcdeps.ExpandDirectories(