Improved error logging for incompatible plugin versions.

Version incompatibility errors were being suppressed by the logic around only logging the error in `firstException`. If you're loading a plugins folder, the error thrown when trying to load that folder *as a plugin* will always be the `firstException`, so you miss any subsequent errors.

Also, the %s tokens were not being swapped out in `generator.js`, so made the line use plain ol' string concatenation.