Remote shell: Print stderr if Python exits unexpectedly

When importing "dogtail.tree" when the AT SPI bus was inaccessible, the
Python process was terminated via SIGTRAP:

    (.:3116166): dbind-ERROR **: 14:35:52.927: AT-SPI: Couldn't connect to accessibility bus. Is at-spi-bus-launcher running?

That was hard to debug because we didn't print the stderr of the failed
Python command. With this commit, failures like the one above should be
a lot easier to debug.