marshal: Add an error when UUID has wrong length (#890)

* marshal: Add expectations to errors for Marshaling

What
===
Add explicit expectations to exactly what error is returned in
test cases where the marshal and unmarshal results in an error.

Why
===
In cases where an unmarshal fails the presence of an error was expected
but not what error was returned. Marshal is not tested for the cases
where it fails in the table tests. Both of these are needed to add
more tests that test for new errors being returned.

* marshal: Add an error when UUID has wrong length

What
===
Add a descriptive error when a UUID is attempted to be marshaled or
unmarshaled and the length of the []byte is not exactly 16 bytes long.

Example
===

```
can not marshal []byte 6 bytes long into timeuuid, must be exactly 16 bytes long
```

Why
===
In cases where UUID []byte was not the correct length the generic `can
not marshal %T into %s` error was being returned. The error was
misleading because it gave the impression the root cause was the type of
the data and not it's length. Additionally the `%T` formatter for a
`[]byte` prints out `[]uint8` which was even more confusing.

* authors: Add Leigh McCulloch to AUTHORS

What
===
Add `Leigh McCulloch` to AUTHORS.

Why
===
As part of his first contribution.

AUTHORS

@@ -86,3 +86,4 @@ Bo Blanton <bo.blanton@gmail.com>
 Vincent Rischmann <me@vrischmann.me>
 Jesse Claven <jesse.claven@gmail.com>
 Derrick Wippler <thrawn01@gmail.com>
+Leigh McCulloch <leigh@leighmcculloch.com>

marshal.go

@@ -1423,9 +1423,10 @@ func marshalUUID(info TypeInfo, value interface{}) ([]byte, error) {
 	case UUID:
 		return val.Bytes(), nil
 	case []byte:
-		if len(val) == 16 {
-			return val, nil
+		if len(val) != 16 {
+			return nil, marshalErrorf("can not marshal []byte %d bytes long into %s, must be exactly 16 bytes long", len(val), info)
 		}
+		return val, nil
 	case string:
 		b, err := ParseUUID(val)
 		if err != nil {