exception: Add exceptions when context creation fails

* Exceptions only fired in constructor when it would initialise with a NULL
  context causing segfaults if used any further
* Adds exception.i requirement to mraa.i for node.js and python support

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>

examples/python/aio.py

@@ -25,5 +25,9 @@
 import mraa
 
 print (mraa.getVersion())
-x = mraa.Aio(0)
-print (x.read())
+
+try:
+    x = mraa.Aio(0)
+    print (x.read())
+except:
+    print ("Are you sure you have an ADC?")