tools: gpio: Print error string on IOCTL failures

Add to error messages the error description by concatenating
output of strerror() function to error messages print out by
gpio-utils.c on IOCTL failures.
Rationalize error messages, while at there, making all of them
look the same.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>