markup: allow better lxml import failure detection.

- in some conditions previous syntaxt could cause errors in first import.

rhodecode/lib/markup_renderer.py

@@ -77,6 +77,13 @@ def relative_links(html_source, server_path):
     if not html_source:
         return html_source
 
+    try:
+        from lxml.html import fromstring
+        from lxml.html import tostring
+    except ImportError:
+        log.exception('Failed to import lxml')
+        return html_source
+
     try:
         doc = lxml.html.fromstring(html_source)
     except Exception: