Clarify error messages in roll_dep.py

roll_dep.py

 #!/usr/bin/env python
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Rolls DEPS controlled dependency.
 
 Works only with git checkout and git dependencies.  Currently this
 script will always roll to the tip of to origin/master.
 """
@@ skipping 60 matching lines @@
          ignore_dirty_tree=False):
   deps = os.path.join(root, 'DEPS')
   try:
     with open(deps, 'rb') as f:
       deps_content = f.read()
   except (IOError, OSError):
     raise Error('Ensure the script is run in the directory '
                 '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:
     print('Warning: %s is not checked out at the expected revision in DEPS' %
           deps_dir)
     if key is None:
       print("Warning: no key specified.  Using '%s'." % deps_dir)
       key = deps_dir
 
     # It happens if the user checked out a branch in the dependency by himself.
@@ skipping 117 matching lines @@
 
   except Error as e:
     sys.stderr.write('error: %s\n' % e)
     return 1
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())