Better handling of disabling modules missing on target platforms

While it's been mentioned a number of times before (ref: #1182884: Platform comparisons may be stale, result in broken migration & #956998: rename (and perhaps clone) should not invoke provision-deploy (or avoid invoking drush updatedb)), we sometimes see modules de-activated (almost) silently when a site is migrated to a platform that's missing an active module. I believe this behaviour is in Drupal core itself, and no doubt for good reasons.

However, when mass migrating sites from one platform to another, there is little indication that such a thing has occurred. You basically need to go through the migration task logs one at a time looking for warnings. Aegir will flag these as warnings before-hand, when migrating an individual site, and in platform comparisons. Note that this requires a verification of the site, to ensure its list of active modules is up-to-date.

After some discussion at Koumbit, we've come up with several alternatives. Most would be complementary, rather then mutually exclusive, but any one would help solve this problem.

1. Implement a "strict mode" (on sites), that would flag active modules that are missing in the target platform as errors, rather than warnings. This should, in turn, block migration until the problem is resolved, or strict mode is disabled for the site.
2. On completion of any migration that disables modules, have the task show up as a warning (yellow) in the task queue, indicating that it needs attention (implemented in: #1929372: Flag tasks with logged warnings)
3. Provide a 'compare platforms' feature for mass migrations. Instead of just listing potential target platforms, also list how many sites would potentially have errors and warnings. Then, present a list of sites, presented similarly to 'compare platforms', showing which sites would have errors or warnings. One could then choose which sites to migrate from that list.

We'd welcome feedback from others on this topic, whether it be preferences or implementation suggestions regarding the above options, or other alternatives.