Fixed form cleaning behavior upon error

When an error occurs while processing the form's data, cleaning up the
data can result in attempting to delete non existent keys. This patch
should make the code tolerant to the half-created form data we have to
deal with in that case.

The problem happened 5 or 6 times within the last 3 weeks.

www/maposmatic/forms.py

@@ -112,7 +112,8 @@ class MapRenderingJobForm(forms.ModelForm):
                 if val is None:
                     msg = _(u"Required")
                     self._errors['bbox'] = forms.util.ErrorList([msg])
-                    del cleaned_data[f]
+                    if f in cleaned_data:
+                        del cleaned_data[f]
 
             # Make sure that bbox and admin modes are exclusive
             cleaned_data["administrative_city"] = ''
@@ -121,7 +122,7 @@ class MapRenderingJobForm(forms.ModelForm):
             # Don't try to instanciate a bounding box with empty coordinates
             if self._errors:
                 return cleaned_data
-                
+
             lat_upper_left = cleaned_data.get("lat_upper_left")
             lon_upper_left = cleaned_data.get("lon_upper_left")
             lat_bottom_right = cleaned_data.get("lat_bottom_right")