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>

api/mraa/uart.hpp

@@ -25,6 +25,7 @@
 #pragma once
 
 #include "uart.h"
+#include <stdexcept>
 
 namespace mraa {
 
@@ -43,6 +44,10 @@ class Uart {
          */
         Uart(int uart) {
             m_uart = mraa_uart_init(uart);
+
+            if (m_uart == NULL) {
+                throw std::invalid_argument("Error initialising UART");
+            }
         }
         /**
          * Uart destructor