ENH: Added file path existence check for read_hdf
closes  #7715 

Added simple check for file path existence. It might be more advisable to deprecate this function and separate file buffer and file path read functionality, but the current form matches the rest of the API.

I wasn't sure if there was any tests I needed to add, but if there is, let me know.

needs a test. You need to check that this raises appropriately..
 Okay, where should the test be placed? Or should I create a new test file?
 `pandas/io/tests/test_pytables.py`

you can create a new function; model off some of the other ones (e.g. search for `read_hdf` and put it somewhere after)
 also will need a release note in v0.15.0.txt API section
 Oh duh, sorry. I was looking in the wrong place for tests. Will do! Thanks.
 minor comment, pls squash as well. looks good otherwise. ping when ready/green.
 @jreback I think everything should be good. Let me know if I need to fix anything else. Thanks for your patience! :)
 thanks!
