Hardware baud rates are fixed by the available divisor ratios in the chip hardware, in combination with the crystal frequency used. And for some crystal values, the available divisor ratios mean you cannot achieve some baud rates, so its worth displaying the error as a warning. Hence its possible and useful to calculate the baud rate error.
For a software UART, the timing is done purely by computing suitable delays ( software loops ) within the software, so in principle you should not get any significant error. So there is not much point in displaying it. There will of course be residual round-off errors, but for low baud rates, these should not give any transmission errors.
Baud rate errors of up to 2% or so should not result in transmission errors, regardless of whether you use hardware or software UARTs.
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