adb: fix GetFeatureSet() to indicate failure.

Previously GetFeatureSet() on an invalid target would just return an
empty feature set, leading to some invalid assumptions, e.g. if there
isn't exactly one device connected this happens:

$ adb shell -t
error: target doesn't support PTY args -Tt

This CL adds a success/failure return value to GetFeatureSet(), and
also adds an option to print errors to stderr since that's the most
common behavior.

This will cause a slight difference in behavior for install/uninstall.
Previously they would block until the device was available, now they
print an error and quit immediately, which seems to be the more common
behavior for adb functions.

Bug: http://b/26387641
Change-Id: I0ea6ffaec922e04b9946e84f05c3870e5b549fde