Implemented multi-error handling in MultiError() class (#614)

* Implemented multi-error handling in MultiError() class

* Added newsfragement file and test

* updated doc-string and list-struct

* updating docstring

tests/exceptions/test_exceptions.py

@@ -0,0 +1,13 @@
+from libp2p.exceptions import (
+    MultiError,
+)
+
+
+def test_multierror_str_and_storage():
+    errors = [ValueError("bad value"), KeyError("missing key"), "custom error"]
+    multi_error = MultiError(errors)
+    # Check for storage
+    assert multi_error.errors == errors
+    # Check for representation
+    expected = "Error 1: bad value\n" "Error 2: 'missing key'\n" "Error 3: custom error"
+    assert str(multi_error) == expected