BufferCore wastes time in canTransform() creating error message that is never used

[peci1]

extrapolation_error_string

geometry2/tf2/src/buffer_core.cpp

Line 361 in 0355229

CompactFrameID parent = f.gather(cache, time, &extrapolation_error_string);

is only read when error_string is non-null:

geometry2/tf2/src/buffer_core.cpp

Lines 461 to 466 in 0355229

	if (error_string)
	{
	std::stringstream ss;
	ss << extrapolation_error_string << ", when looking up transform from frame [" << lookupFrameString(source_id) << "] to frame [" << lookupFrameString(target_id) << "]";
	*error_string = ss.str();
	}

However, even when error_string is null, pointer to extrapolation_error_string is still passed to gather(), which means the stringstream operations are performed no matter what the value of error_string is.

And the string operations aren't exactly cheap (this is a flamegraph from a profiler, x axis is total time accumulated over all calls to the function):

[peci1]

Okay, this simple case is solved by #469. But there's also a great performance loss due to the error string construction in lookupTransform where you can't choose to not construct the strings. I'm just thinking about an interface that would allow to tell that the user is not interested in the error string, but just in the exception type...