Ensure that problems installing NEST extensions produce only a warnin…

…g, not an error.

doc/installation.txt

@@ -22,7 +22,7 @@ Optional dependencies are:
 Installing PyNN
 ===============
 
-.. note:: if using NEURON, it is easiest if you install NEURON *before* you install PyNN (see below).
+.. note:: if using NEURON or NEST, it is easiest if you install NEURON/NEST *before* you install PyNN (see below).
 
 The easiest way to get PyNN is to use pip_::
 
@@ -49,6 +49,8 @@ Test it using something like the following::
     >>> sim.end()
 
 (This assumes you have NEST installed).
+If you get a warning "Unable to install NEST extensions. Certain models may not be available" then ensure the
+program :command:`nest-config` is on your system PATH.
 
 With NEURON as the simulator, make sure you install NEURON *before* you install PyNN.
 The PyNN installation will then compile PyNN-specific membrane mechanisms, which are loaded when importing the :mod:`neuron` module::

pyNN/nest/__init__.py

@@ -7,6 +7,7 @@
 
 """
 
+import warnings
 import numpy
 try:
     import tables  # due to freeze when importing nest before tables
@@ -47,7 +48,10 @@
 if logger.level == logging.NOTSET:
     logger.setLevel(logging.ERROR)
 
-nest.Install('pynn_extensions')
+try:
+    nest.Install('pynn_extensions')
+except nest.NESTError as err:
+    warnings.warn("Unable to install NEST extensions. Certain models may not be available.\nFurther details: {}".format(err))
 
 
 # ==============================================================================