printf("Please enter the number's base:n");
if(scanf("%d", &baseIn) != 1){
    printf("An error occurred!n");
    return 0;
}
if(baseIn < 2 || baseIn > 16){
    printf("Invalid input basen");
    return 0;
}

printf("Please enter the desired base:n");
if(scanf("%d", &baseOut) != 1){
    printf("An error occurred!n");
    return 0;
}
if(baseOut < 2 || baseOut > 16){
    printf("Invalid desired basen");
    return 0;
}
...
(More less relevant code)

if(scanf("%d", &baseIn) != 1){
    printf("An error occurred!n");
    return 0;
}

if(scanf("%d", &baseIn) <= ){
    printf("An error occurred!n");
    return 0;
}