Raise better IOError if a connection file is not found

when starting parallel.Client

Example:

    IOError: Connection file u’~/.ipython/profile_default/security/ipcontroller-client.json' not found. Is a controller running?
even better IOError for no connection file
Merge pull request #6087 from minrk/better-error-no-connection-file

Raise better IOError if a connection file is not found