Error -2
Can occur due to NAND Failure or iPhone 7/7 Plus Baseband/Audio IC Failure or in rare circumstances an ever so slightly improperly connected battery. Generally something hardware related.
Error -1
Error occurs when one uses the "last ditch" method for preventing baseband updates on the iPhone 4. The baseband does not get updated using this method. Use TinyUmbrella or redsn0w to kick it out of Recovery Mode.
Also occurs in restore logs as "unable to convert ramrod 1004." Seemingly caused by a modem firmware chipset malfunction or faulty dock connector, the only fix is to either heat up the chipset or replace the dock connector.