Use passed-in info argument for TraitType.error() (#884)

Author: kylebarron

traitlets/traitlets.py

@@ -848,13 +848,13 @@ def error(self, obj, value, error=None, info=None):
                     e = "The '{}' trait of {} instance expected {}, not {}.".format(
                         self.name,
                         class_of(obj),
-                        self.info(),
+                        info or self.info(),
                         describe("the", value),
                     )
                 else:
                     e = "The '{}' trait expected {}, not {}.".format(
                         self.name,
-                        self.info(),
+                        info or self.info(),
                         describe("the", value),
                     )
                 raise TraitError(e)