When the registerXX() function hasn't been called as a result of not linking in the library containing the corresponding MONGO_INITIALIZER, a std::bad_function_call exception is thrown that doesn't provide any useful context toward remediation steps.

Note: I'm filing this improvement request on behalf of xiangyu.yao; he encountered this issue while trying to define a target for C++ unit tests of the AutoGetXX RAII types. It would have been nice if the exception provided more context to have immediately realized that a library dependency was missing.