Better error code for unmounting not-mounted volumes

ENOENT is more descriptive than the generic error that EINVAL gets
turned into. Then we can actually treat it how we want on the other
side.

Change-Id: I9b4f3be6308e13f680eae368d2167ab9ee6aae5f

VolumeManager.cpp

@@ -559,7 +559,7 @@ int VolumeManager::unmountLoopImage(const char *id, const char *idHash,
         const char *fileName, const char *mountPoint, bool force) {
     if (!isMountpointMounted(mountPoint)) {
         SLOGE("Unmount request for %s when not mounted", id);
-        errno = EINVAL;
+        errno = ENOENT;
         return -1;
     }