Make script ignore encoding troubles in dropbox API.

* Catch any UnicodeError and write notification to stderr
* Fixes with `decode(..., error='ignore')` might break data and is therefore not recommended
* Dropbox API does not specify encoding for filenames (which is an everlasting open OS issue)
* `sys.stderr` should be the most py3k/python2-compatible way to write to stderr
* Error message does not mention affected filename to avoid encoding issues in error message