Move __adb_error to std::string, and improve various errors.

Also remove an sprintf. Also fix various bits of code that were
reporting stale adb_error values when they meant strerror.

(cherry-pick of 078f0fcf.)

Bug: http://b/20666660
Change-Id: Ibeb48b7bc21bb0ec30ba47889d1d671ee480e1b7