use safe_str to serialize Exceptions Fixes #5478

Author: graingert

src/_pytest/_code/code.py

@@ -572,8 +572,9 @@ def match(self, regexp):
         raised.
         """
         __tracebackhide__ = True
-        if not re.search(regexp, str(self.value)):
-            assert 0, "Pattern '{!s}' not found in '{!s}'".format(regexp, self.value)
+        value = safe_str(self.value)
+        if not re.search(regexp, value):
+            assert 0, "Pattern {!r} not found in {!r}".format(regexp, value)
         return True