Log the exception when an invalid modulemd is encountered

This will help us determine why a modulemd couldn't be parsed
other than formatting issues. An example of this is if the database
session becomes stale, the exception is not due to the modulemd being
invalid.

module_build_service/models.py

@@ -384,6 +384,7 @@ class ModuleBuild(MBSBase):
             mmd = Modulemd.Module().new_from_string(self.modulemd)
             mmd.upgrade()
         except Exception:
+            log.exception('An error occurred while trying to parse the modulemd')
             raise ValueError("Invalid modulemd")
         return mmd