Clarify error messages in roll_dep.py

roll_dep.py

Index: roll_dep.py
diff --git a/roll_dep.py b/roll_dep.py
index 6cae24f388f66ea271b4d0d783fb12966161bd6c..e40f55d4fa25a9865a97c5fada634ec375575f73 100755
--- a/roll_dep.py
+++ b/roll_dep.py
@@ -78,11 +78,11 @@ def roll(root, deps_dir, roll_to, key, reviewers, bug, no_log, log_limit,
                 'containing DEPS file.')
 
   if not ignore_dirty_tree and not is_pristine(root):
-    raise Error('Ensure %s is clean first.' % root)
+    raise Error('Ensure %s is clean first (no non-merged commits).' % root)
 
   full_dir = os.path.normpath(os.path.join(os.path.dirname(root), deps_dir))
   if not os.path.isdir(full_dir):
-    raise Error('Directory not found: %s' % deps_dir)
+    raise Error('Directory not found: %s (%s)' % (deps_dir, full_dir))
   head = check_output(['git', 'rev-parse', 'HEAD'], cwd=full_dir).strip()
 
   if not head in deps_content: