Improve "[PARAM] is a required [TYPE]" message

This happens when Guzzle is trying to create an error message of the
form "ID is a required integer", but the type is an array (union) of
types rather than a single type in a string.

In PHP 5.4, this causes a notice "array to string conversion", and in
all versions of PHP, the message Guzzle generated reads "ID is a
required Array" (where "Array" is the result of the array-to-string
conversion, almost always not what you want).
Merge pull request #272 from bradfeehan/improve-require-message

Improve "[PARAM] is a required [TYPE]" message