Fixed Import error due to pre tag

Fixed regex for nested pre tags

Updated import export test

fixed quality test issue

fixed No exception type(s) specified

updated tests

fixed linter issue

fixed linter issue

fixed linter issue

common/lib/xmodule/xmodule/raw_module.py

@@ -1,5 +1,3 @@
-
-
 import logging
 import re
 
@@ -12,7 +10,7 @@ from .exceptions import SerializationError
 
 log = logging.getLogger(__name__)
 
-PRE_TAG_REGEX = re.compile(r'<pre>[\s\S]*?</pre>')
+PRE_TAG_REGEX = re.compile(r'<pre\b[^>]*>(?:(?=([^<]+))\1|<(?!pre\b[^>]*>))*?</pre>')
 
 
 class RawMixin(object):
@@ -25,12 +23,21 @@ class RawMixin(object):
 
     @classmethod
     def definition_from_xml(cls, xml_object, system):
-        pre_tag_data = [etree.tostring(pre_tag_info) for pre_tag_info in xml_object.findall('.//pre')]
-        data = etree.tostring(xml_object, pretty_print=True, encoding='unicode')
-        if pre_tag_data:
-            for index, pre_tag in enumerate(re.findall(PRE_TAG_REGEX, data)):
-                data = re.sub(pre_tag, pre_tag_data[index].decode(), data)
-        return {'data': data}, []
+        try:
+            data = etree.tostring(xml_object, pretty_print=True, encoding='unicode')
+            pre_tag_data = []
+            for pre_tag_info in xml_object.findall('.//pre'):
+                if len(pre_tag_info.findall('.//pre')) == 0:
+                    pre_tag_data.append(etree.tostring(pre_tag_info))
+
+            if pre_tag_data:
+                matches = re.finditer(PRE_TAG_REGEX, data)
+                for match_num, match in enumerate(matches):
+                    data = re.sub(match.group(), pre_tag_data[match_num].decode(), data)
+            etree.XML(data)  # it just checks if generated string is valid xml
+            return {'data': data}, []
+        except etree.XMLSyntaxError:
+            return {'data': etree.tostring(xml_object, pretty_print=True, encoding='unicode')}, []
 
     def definition_to_xml(self, resource_fs):
         """