Currently when preprocessing step fails item becomes notsupported with message like:
Item preprocessing step #<N> failed: <error description>

However often the problem is in the previous steps - adding a 'log' of executed steps would be helpful. The proposed format is:
Preprocessing failed for: <value>
1. <success|fail>(<custom on fail action>): <value:error>
...

Value will be truncated to 50 symbols, control characters replaced with '.'.