Add check for missing unidiff dependency

Author: thk123

scripts/diff_to_added_lines.py

@@ -4,7 +4,12 @@
 
 def diff_to_added_lines(diff_file, repository_root, out_stream):
 
-  import unidiff
+  try:
+    import unidiff
+  except ImportError:
+    print("diff_to_added_lines.py requires unidiff, use `pip install --user unidiff` to install")
+    sys.exit(1)
+
   import os.path
   import json