Calls to bson_append_* are not checked for errors in the core builder

Throughout the implementation of the core builder (in src/bsoncxx/builder/core.cpp), there are calls to various `bson_append_*` functions, all of which return a boolean indicating whether the call was successful. We should check the return value of these calls and throw an appropriate error message in the case that that it's false, as currently all of them ignore the value returned/