Detect unchecked error within SimpleStmt of an IfStmt with error checking in conditional

[dsnet]

(I'm aware of #1057, but I believe I'm reporting a particular unchecked error that has low false positives)

Consider a code that looks like the following:

if foo.WriteEntry(...); err != nil {
    ...
}

This is a bug, and the correct code should have been:

if err := foo.WriteEntry(...); err != nil {
    ...
}

I propose flagging the combination of:

1. an unchecked error value in the SimpleStmt of the IfStmt
2. an error check of some kind within the Expression of the IfStmt

Condition 2 almost certainly indicates a programmer's intent to do error checks, which should drastically reduce any false positives with condition 1.