Communicating migration results

When each step (configuration and content) of an upgrade migration is run, what do we tell the user about the results and how?

The current recommendation is that the immediate feedback be very simple - like, 53 configuration items have been imported - with a link to a log of the details. As a first pass, we could simply log things to the watchdog and link to the filtered dblog view - ah, but what if they're using syslog? So, we probably want a custom logging solution.

What precisely should be logged? The simplest thing as a first pass would be a log entry per migration - the next level of refinement would be to log per module (source module or destination module?). It would be good to log counts where it makes sense - i.e., for things like number of vocabularies or nodes imported, but not for configuration variables.

Until a consensus emerges, I plan on updating the ui_poc branch as follows:

1. Dumping basic per-migration feedback into watchdog, just to get the feel of having logging separated from the immediate feedback.
2. Immediate feedback of number of migrations successfully completed.