Fix issues reported by `golangci-lint`

[dveeden]

$ golangci-lint run
generate_perror/main.go:148:11: Error return value of `cmd.Wait` is not checked (errcheck)
        cmd.Wait()
                ^
src/main.go:294:13: Error return value of `t.mdb.Exec` is not checked (errcheck)
        t.mdb.Exec(fmt.Sprintf("drop database `%s`", strings.ReplaceAll(t.name, "/", "__")))
                  ^
src/main.go:921:18: Error return value of `t.curr.conn.Raw` is not checked (errcheck)
        t.curr.conn.Raw(func(driverConn any) error {
                       ^
generate_perror/main.go:173:9: S1005: unnecessary assignment to the blank identifier (gosimple)
    for i, _ := range codes {
           ^
generate_perror/main.go:81:6: S1009: should omit nil check; len() for []string is defined as zero (gosimple)
        if m != nil && len(m) == 3 && m[1] != "" && m[2] != "" {
           ^
generate_perror/main.go:137:7: S1009: should omit nil check; len() for []string is defined as zero (gosimple)
            if m != nil && len(m) == 3 && m[1] != "" && m[2] != "" {

[Defined2014]

@bb7133 PTAL