[HatoholException] Make it easy to understand the reason of an build error.

For example, the follwoing error message is shown when the exp != val.

HatoholException.h: In instantiation of 'T _buildExpect(T) [with T = long unsigned int; bool COND = false]':
DBClientConfig.cc:116:3:   required from here
HatoholException.h:92:2: error: invalid use of incomplete type 'struct _BuildError'
  _BuildError("The used value is invalid (unexpected) or the matched specialized template method is not defined.");
  ^
HatoholException.h:87:8: error: forward declaration of 'struct _BuildError'
 struct _BuildError; // The body is not defined to cause an build error
[HatoholException] Fix a compiler error on clang.
[HatoholException] Revise a comment of HATOHOL_BUILD_EXPECT().
Merge pull request #371 from project-hatohol/hatohol-build-expect-easy-understand

[HatoholException] Make it easy to understand the reason of an build err...