Throw OutOfSpaceException instead of IllegalArgumentException.

When a MessageNano containing a String is serialized into a buffer that
is too small to contain it, and the buffer's boundary happens to be
where the string field's length delimiting varint is serialized,
and the string's length and 3*length have the same length when
encoded as a varint, an IllegalArgumentException is thrown rather than
an OutOfSpaceException.

Github issue: https://github.com/google/protobuf/issues/292

Change-Id: If478d68cf15bfd0662252d008e42b2bf1ff1c75e