Bugfix: craft vehicle welding rig with two welders, less message spam, more messages

Fix the tool-count vs tool-charges confusion in the crafting requirements, reported there http://smf.cataclysmdda.com/index.php?topic=7319 - tool charges are requested by a positive number, tool counts by a negative number.

Show message from acid damage even with minor damage. Currently the message is only shown when the field density is 3, for lower density the damage is applied, but no message is printed.

Fixes #7134 (only one "book is too complex" message) - the message for recipes that could be learned and the message on how to stop has the same check right above.