Capture exception details in AWS Lambda middleware. (#154)

Trace field names match the ones used in Tracer:
beeline-python/beeline/trace.py

```python
span.add_context({ 
    "app.exception_type": str(type(e)), 
    "app.exception_string": stringify_exception(e), 
    "app.exception_stacktrace": traceback.format_exc(), 
}) 
```
closes #153

Author: fitzoh

beeline/middleware/awslambda/__init__.py

@@ -1,3 +1,5 @@
+import traceback
+
 import beeline
 from beeline.propagation import Request
 # In Lambda, a cold start is when Lambda has to spin up a new instance of a
@@ -146,6 +148,13 @@ def _beeline_wrapper(event, context):
                 beeline.add_context_field('app.response', resp)
 
             return resp
+        except Exception as e:
+            beeline.add_context({
+                "app.exception_type": str(type(e)),
+                "app.exception_string": beeline.internal.stringify_exception(e),
+                "app.exception_stacktrace": traceback.format_exc(),
+            })
+            raise e
         finally:
             # This remains false for the lifetime of the module
             COLD_START = False

beeline/middleware/awslambda/test_awslambda.py

@@ -203,6 +203,37 @@ def handler(event, context):
                 'name': 'handler'}, ANY)
             m_add_context_field.assert_called_once_with('app.response', 1)
 
+    def test_handle_exceptions(self):
+        ''' ensure instrumentation occurs when the handler raises an exception '''
+        with patch('beeline.propagate_and_start_trace') as m_propagate, \
+                patch('beeline.add_context_field') as m_add_context_field, \
+                patch('beeline.add_context') as m_add_context, \
+                patch('beeline.middleware.awslambda.beeline._GBL'), \
+                patch('beeline.middleware.awslambda.COLD_START') as m_cold_start:
+            m_event = Mock()
+            m_context = Mock(function_name='fn', function_version="1.1.1",
+                             aws_request_id='12345')
+
+            @awslambda.beeline_wrapper
+            def handler(event, context):
+                raise ValueError('something went wrong')
+
+            with self.assertRaises(ValueError):
+                handler(m_event, m_context)
+
+            m_propagate.assert_called_once_with({
+                'app.function_name': 'fn',
+                'app.function_version': '1.1.1',
+                'app.request_id': '12345',
+                'app.event': ANY,  # 'app.event' is included by default
+                'meta.cold_start': ANY,
+                'name': 'handler'}, ANY)
+            m_add_context.assert_called_once_with({
+                'app.exception_type': ANY,  # representation changes between 2.7 and 3.x
+                'app.exception_string': 'something went wrong',
+                'app.exception_stacktrace': ANY
+            })
+
     def test_can_omit_input(self):
         ''' ensure input event field can be omitted '''
         with patch('beeline.propagate_and_start_trace') as m_propagate, \