Misleading error messages when opening inexistent json file
When opening a json file that doesn't exist with `read_json`, we get a `ValueError` with error messages like:

```python
>>> import pandas
>>> pandas.read_json('no_file.json')
ValueError: Unexpected character found when decoding 'null'

>>> pandas.read_json('a_dir/no_file.json')
ValueError: Expected object or value
```

It'd probably be better to raise an exception like:
```
FileNotFoundError: No such file: 'no_file.json'
```