Receiving this error using xcode 7 GM with a Notification Scheme using iPhone 6 + Apple Watch 38mm when building and running. IOS app and watch app work in Simulators. Sub message is 'SPErrorUnknownMessage'. Have Reset Content and Settings which results in 'black' screen for watch simulator, then error message in xcode.
Last few messages in System.Log for watch OS2:
gizmoappd[3229]: (Error) WatchKit: Unable to find a native plugin for identifier com.appname.watchapp
--- last message repeated 2 times ---
com.apple.CoreSimulator.SimDevice.4F304B64-16AD-4F3C-82C3-10712F361EBC.launchd_sim[3207] (com.apple.xpc.launchd.domain.pid.(premature birth).0): assertion failed: 14F27: launchd_sim + 114772 [C199BB42-75A4-3253-9467-9D3B6189DEF5]: 0x1
com.apple.CoreSimulator.SimDevice.4F304B64-16AD-4F3C-82C3-10712F361EBC.launchd_sim[3207] (com.apple.xpc.launchd.domain.pid.(premature birth).0): Could not resolve origin of domain. XPC services in this domain's bundle will not be bootstrapped: error = 3: No such process, taint = (untainted)
com.apple.CoreSimulator.SimDevice.4F304B64-16AD-4F3C-82C3-10712F361EBC.launchd_sim[3207] (com.apple.xpc.launchd.domain.pid.(premature birth).0): assertion failed: 14F27: launchd_sim + 114772 [C199BB42-75A4-3253-9467-9D3B6189DEF5]: 0x1
com.apple.CoreSimulator.SimDevice.4F304B64-16AD-4F3C-82C3-10712F361EBC.launchd_sim[3207] (com.apple.xpc.launchd.domain.pid.(premature birth).0): Could not resolve origin of domain. XPC services in this domain's bundle will not be bootstrapped: error = 3: No such process, taint = (untainted)