Prevents crash when a route resolves to a 404 (#129)

multiple issues:
 * resolve() might throw, handle that case
 * unhandled exception in `process_request` leads to last successful
   `request_model` remain in `DataCollector` and being used to create
   a response model for the current failed request, leading to
   a constraint violation thrown by the db (creating another response
   for the previously succesful request)

resolution
 * handle 404 by saving NULL/None as view name
 * clean out `DataCollector` state when starting a request, before doing
   anything.

Might help #26

silk/middleware.py

@@ -85,18 +85,19 @@ def _apply_dynamic_mappings(self):
 
     @silk_meta_profiler()
     def process_request(self, request):
-        request_model = None
-        if _should_intercept(request):
-            Logger.debug('process_request')
-            request.silk_is_intercepted = True
-            self._apply_dynamic_mappings()
-            if not hasattr(SQLCompiler, '_execute_sql'):
-                SQLCompiler._execute_sql = SQLCompiler.execute_sql
-                SQLCompiler.execute_sql = execute_sql
-            request_model = RequestModelFactory(request).construct_request_model()
-            DataCollector().configure(request_model)
-        else:
-            DataCollector().clear()
+        DataCollector().clear()
+
+        if not _should_intercept(request):
+            return
+
+        Logger.debug('process_request')
+        request.silk_is_intercepted = True
+        self._apply_dynamic_mappings()
+        if not hasattr(SQLCompiler, '_execute_sql'):
+            SQLCompiler._execute_sql = SQLCompiler.execute_sql
+            SQLCompiler.execute_sql = execute_sql
+        request_model = RequestModelFactory(request).construct_request_model()
+        DataCollector().configure(request_model)
 
     @transaction.atomic()
     def _process_response(self, request, response):

silk/model_factory.py

@@ -5,7 +5,7 @@
 import base64
 from uuid import UUID
 
-from django.core.urlresolvers import resolve
+from django.core.urlresolvers import resolve, Resolver404
 
 from silk import models
 from silk.collector import DataCollector
@@ -155,20 +155,30 @@ def query_params(self):
             encoded_query_params = json.dumps(query_params_dict)
         return encoded_query_params
 
-    def construct_request_model(self):
-        body, raw_body = self.body()
-        query_params = self.query_params()
-        path = self.request.path
-        resolved = resolve(path)
+    def view_name(self):
+        try:
+            resolved = resolve(self.request.path)
+        except Resolver404:
+            return None
+
         try:
             # view_name is set in Django >= 1.8
-            view_name = resolved.view_name
+            return resolved.view_name
         except AttributeError:
             # support for Django 1.6 and 1.7 in which no view_name is set
             view_name = resolved.url_name
             namespace = resolved.namespace
             if namespace:
                 view_name = namespace + ':' + view_name
+
+            return view_name
+
+    def construct_request_model(self):
+        body, raw_body = self.body()
+        query_params = self.query_params()
+        path = self.request.path
+        view_name = self.view_name()
+
         request_model = models.Request.objects.create(
             path=path,
             encoded_headers=self.encoded_headers(),