make all exceptions inherit from ipfsApiError
make all exceptions inherit from ipfsApiError

this change makes all exceptions in `ipfsApi.exceptions` inherit from
`ipfsApiError`.  This allows client to differenate between errors raised
explicitly by the ipfsApi module vs. those stemming from other problems:

    try:
        api.cat('...')
    except ipfsApiError as err:
        ...response to ipfs error...

alexander255/py-ipfs-api,ipfs/python-ipfs-api,moreati/python-ipfs-api,alexander255/py-ipfs-api,ipfs/py-ipfs-api,ipfs/py-ipfs-api