Raise ValidationError on PyXFormError exceptions

This should allow the user to see what the problem with the merged form
is.

onadata/libs/serializers/merged_xform_serializer.py

@@ -15,6 +15,7 @@
 from onadata.apps.logger.models.xform import XFORM_TITLE_LENGTH
 from onadata.libs.utils.common_tags import MULTIPLE_SELECT_TYPE, SELECT_ONE
 from pyxform.builder import create_survey_element_from_dict
+from pyxform.errors import PyXFormError
 
 SELECTS = [SELECT_ONE, MULTIPLE_SELECT_TYPE]
 
@@ -179,7 +180,11 @@ def create(self, validated_data):
         survey['sms_keyword'] = survey['id_string']
         survey['title'] = validated_data.pop('name')
         validated_data['json'] = survey.to_json()
-        validated_data['xml'] = survey.to_xml()
+        try:
+            validated_data['xml'] = survey.to_xml()
+        except PyXFormError as error:
+            raise serializers.ValidationError(_(
+                "Problem Merging the Form: {}".format(error)))
         validated_data['user'] = validated_data['project'].user
         validated_data['created_by'] = request.user
         validated_data['is_merged_dataset'] = True