pd.read_json cryptic error on missing file
``` python

In [12]: pd.read_json("No Such File")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-12-f479cae2f65a> in <module>()
----> 1 pd.read_json("No Such File")

/home/user1/src/pandas/pandas/io/json.pyc in read_json(path_or_buf, orient, typ, dtype, convert_axes, convert_dates, keep_default_dates, numpy, precise_float, date_unit)
    188         obj = FrameParser(json, orient, dtype, convert_axes, convert_dates,
    189                           keep_default_dates, numpy, precise_float,
--> 190                           date_unit).parse()
    191 
    192     if typ == 'series' or obj is None:

/home/user1/src/pandas/pandas/io/json.pyc in parse(self)
    256 
    257         else:
--> 258             self._parse_no_numpy()
    259 
    260         if self.obj is None:

/home/user1/src/pandas/pandas/io/json.pyc in _parse_no_numpy(self)
    473         if orient == "columns":
    474             self.obj = DataFrame(
--> 475                 loads(json, precise_float=self.precise_float), dtype=None)
    476         elif orient == "split":
    477             decoded = dict((str(k), v)

ValueError: Expected object or value
```

read_json interprets strings which are not filenames as json data, then fails to parse them
if the filename names a path that doesn't exist (due to typo, or being in wrong directory for example).

That overloading makes it <del>impossible</del> nasty to distinguish two distinct error cases, e.g. 
missing file and malformed json. Dubious API choice to my tastes.

in any case, catch both errors and return a saner message "missing file or malformed input" etc.
