Error message from multi-ifo pycbc_live file isn't great

When trying to read a pycbc_live HDF5 file using EventTable.read an exception gets raised when the file contains data for multiple interferometers, but the IFO wasn't specified by the user. The current error message is:

ValueError: PyCBC live HDF5 file contains multiple IFO groups, please select ifo manually

It should really be something like:

ValueError: PyCBC live HDF5 file contains data for multiple interferometers, please specify the prefix of the relevant interferometer via the `ifo` keyword argument, e.g: `ifo=G1`