client.screencap() fails on some 2.3 devices

[sorccu]

Some 2.3 devices print "screencap: permission denied" to stdout when the binary is missing. The current ScreencapCommand doesn't account for that, expecting the output to go to stderr.

[lalith-b]

It also shows only a black screen on the android emulator, when the option is chosen to use host gpu for emulation