plugin XEP-0106: log rejected value when escaping fails.

sat/plugins/plugin_xep_0106.py

@@ -87,7 +87,7 @@ class XEP_0106(object):
         """
         if not escaped or escaped.startswith(r'\27') or escaped.endswith(r'\27'):
             raise ValueError("escaped value must not be empty, or start or end with a "
-                             "whitespace")
+                             f"whitespace: rejected value is {escaped!r}")
         unescaped = []
         idx = 0
         while idx < len(escaped):