Fix XML validation check for mxformat script (AcademySoftwareFoundation#2963)

Update AcademySoftwareFoundation#2956 

Fix so that XML validate returns an error condition. Would previously return nothing so would always pass.

Author: kwokcb

python/Scripts/mxformat.py

@@ -16,6 +16,7 @@ def is_well_formed(xml_string):
         ET.fromstring(xml_string)  
     except ET.ParseError as e:
         error = str(e)
+    return error
 
 def main():
     parser = argparse.ArgumentParser(description="Reformat a folder of MaterialX documents in place.")