For some time now, error reporting from cm_transition() was broken.
Typical symptom was when starting a run from mhttpd, when a transition error occurred, the run does not
start (good) but the user is presented with a message "Success" in big letters (confusing the user).
Part of the problem was caused by user-written frontends that return an empty error string. Code in
cm_transition() now detects this and shows the numeric value of the error status returned by the frontend.
This is fixed in revision 3681.
The error string "Success" is now returned only when cm_transition() was successful, and other error
reporting inside this function was cleaned up.