Log a warning when user tries to add a trace callback but hasn't set a closure-define

[danielcompton]

re-frame.trace/register-trace-cb doesn't validate whether the closure define to enable tracing has been set or not. This could lead to confusing situations where people are trying to use re-frame's tracing abilities, but haven't set the closure-define so no traces show up.

The fix for this is to check at re-frame.trace/register-trace-cb whether tracing is enabled or not, and log a warning if it isn't, telling the user that they need to set the closure define for re-frame.trace.trace-enabled?.

[superstructor]

Fixed in a previous release. See this code. Closing.