Factor out getting unit by checksum

The code was duplicated in two places, having just one in unit manager
is saner.

trans/models/unit.py

@@ -395,6 +395,22 @@ class UnitManager(models.Manager):
             translation__language=unit.translation.language
         )
 
+    def get_checksum(self, request, translation, checksum):
+        '''
+        Returns unit based on checksum.
+        '''
+        try:
+            return Unit.objects.filter(
+                checksum=checksum,
+                translation=translation
+            )[0]
+        except Unit.DoesNotExist:
+            weblate.logger.error('message %s disappeared!', checksum)
+            messages.error(
+                request,
+                _('Message you wanted to translate is no longer available!')
+            )
+            raise
 
 class Unit(models.Model):
     translation = models.ForeignKey(Translation)

trans/views/edit.py

@@ -166,26 +166,12 @@ def handle_translate(obj, request, user_locked, this_unit_url, next_unit_url):
     obj.check_sync()
 
     try:
-        unit = Unit.objects.get(
-            checksum=form.cleaned_data['checksum'],
-            translation=obj
-        )
-    except Unit.MultipleObjectsReturned:
-        # Possible temporary inconsistency caused by ongoing update
-        # of repo, let's pretend everyting is okay
-        unit = Unit.objects.filter(
-            checksum=form.cleaned_data['checksum'],
-            translation=obj
-        )[0]
-    except Unit.DoesNotExist:
-        weblate.logger.error(
-            'message %s disappeared!',
-            form.cleaned_data['checksum']
-        )
-        messages.error(
+        unit = Unit.objects.get_checksum(
             request,
-            _('Message you wanted to translate is no longer available!')
+            obj,
+            form.cleaned_data['checksum'],
         )
+    except Unit.DoesNotExist:
         return
 
     if 'suggest' in request.POST:
@@ -270,26 +256,12 @@ def handle_merge(obj, request, next_unit_url):
         return
 
     try:
-        unit = Unit.objects.get(
-            checksum=mergeform.cleaned_data['checksum'],
-            translation=obj
-        )
-    except Unit.MultipleObjectsReturned:
-        # Possible temporary inconsistency caused by ongoing
-        # update of repo, let's pretend everyting is okay
-        unit = Unit.objects.filter(
-            checksum=mergeform.cleaned_data['checksum'],
-            translation=obj
-        )[0]
-    except Unit.DoesNotExist:
-        weblate.logger.error(
-            'message %s disappeared!',
-            mergeform.cleaned_data['checksum']
-        )
-        messages.error(
+        unit = Unit.objects.get_checksum(
             request,
-            _('Message you wanted to translate is no longer available!')
+            obj,
+            mergeform.cleaned_data['checksum'],
         )
+    except Unit.DoesNotExist:
         return
 
     merged = Unit.objects.get(