snprintf should return a negative number if there is a formatting error.
Limited snprintf implementations may not detect these errors. This
caused the test to fail on the STM32 Discovery F429i.
Change-Id: I1da8593c258508c0d3e2fa6c440a58493ce15597
Provide a string::Format function that writes to a span. The snprintf
return value is interpreted and returned as a StatusWithSize.
Change-Id: Ib7fed684dc333d5feaef85c1bb558cb45e3936bd