Maximum delay is not checked for random distribution
The maximum delay value doesn't seem to be checked when using a RandomDistribution for delays.  The following fails by crashing the delay extension, but it should fail earlier as the high value provided is already out-of-range.

```
import spynnaker8 as sim
sim.setup(0.1)
pop_a = sim.Population(100, sim.IF_curr_exp(i_offset=5.0))
pop_b = sim.Population(100, sim.IF_curr_exp())
exc_delays = sim.RandomDistribution("normal_clipped", mu=1.5*15, sigma=0.75*15, low=0.1*15, high=14.4*15)
proj = sim.Projection(pop_a, pop_b, sim.AllToAllConnector(), sim.StaticSynapse(delay=exc_delays))
sim.run(1000)
sim.end()
```

diff --git a/spynnaker/pyNN/models/pynn_projection_common.py b/spynnaker/pyNN/models/pynn_projection_common.py
index 480d5670df..ace1280345 100644
--- a/spynnaker/pyNN/models/pynn_projection_common.py
+++ b/spynnaker/pyNN/models/pynn_projection_common.py
@@ -119,11 +119,13 @@ class PyNNProjectionCommon(object):
         post_vertex_max_supported_delay_ms = \\
             post_synaptic_population._get_vertex \\
             .get_maximum_delay_supported_in_ms(machine_time_step)
-        if max_delay > (post_vertex_max_supported_delay_ms +
-                        _delay_extension_max_supported_delay):
+        max_supported_delay_ms = post_vertex_max_supported_delay_ms + \\
+            _delay_extension_max_supported_delay * (machine_time_step / 1000.0)
+        if max_delay > max_supported_delay_ms:
             raise ConfigurationException(
-                "The maximum delay {} for projection is not supported".format(
-                    max_delay))
+                "The maximum delay {} for projection is not supported "
+                "(max supported delay is {})".format(max_delay,
+                                                     max_supported_delay_ms))
 
         if max_delay > user_max_delay / (machine_time_step / 1000.0):
             logger.warning("The end user entered a max delay"
['spynnaker/pyNN/models/pynn_projection_common.py']