- Added new exceptions to explain limit over errors.

- Fixed ext maximum size problem.
Problem:
The type of m_trail was uint32_t but when parsing ext, it could be 0xffffffff + 1. +1 means type. See https://github.com/msgpack/msgpack/blob/master/spec.md#ext-format-family

Solution:
Modified the type of m_trail as std::size_t. If sizeof(std::size_t) == 4, 0xffffffff size of ext is an error. If sizeof(std::size_t) == 8, 0xffffffff size of ext is not an error. m_trail is 0xffffffff + 1.

Design cohice:
I chose std::size_t as the m_trail's type instead of uint64_t intentionally. On 64 addressing bit environment, there is no problem. On 32 bit environment, there is no problem except ext with maximum size. There is only one exception in the all msgpack format. Using uint64_t to support that, it's very expensive. On 32 addressing bit environment, allocating 0xffffffff + 1 bytes of memory couldn't succeed, so I believe that throwing an exception is a reasonable design choice in the case.

… std::string& are supported.

Updated all exceptions relate to unpack take a message parameter. It is better to explain the exceptional situation in detail. So far, just passing the exceptions name.