Unable to run SpikeSourceArray forever repeatably

[Christian-B]

to replicate
Add a SpikeSource Array to https://github.com/SpiNNakerManchester/sPyNNaker/blob/master/spynnaker_integration_tests/test_external_devices/test_spike_run_forever_again.py

raised error:
spinn_front_end_common.utilities.exceptions.BufferableRegionTooSmall: The buffer size 61250280 is too small for any data to be added for region 3 of vertex input(0:0)

Better error
Unable to create message for region=<_Regions.SEND_BUFFER: 3> on vertex=input(0:0) while is_empty reports false

[Christian-B]

Better error message
f6acdf1

fix:
4d3e832