FIX: Actually make a function for verifying compile mode

src/main.rs

@@ -44,7 +44,7 @@ fn main() {
         Arg::new("MODE")
             .help("Whether to compile in openmw, morrowind.exe, or librequake mode.")
             .long("mode")
-            .value_parser(validate_input_plugin),
+            .value_parser(validate_compile_mode),
     ])
     .get_matches();
 
@@ -347,6 +347,13 @@ fn validate_input_plugin(arg: &str) -> Result<String, String> {
     Ok(arg.into())
 }
 
+fn validate_compile_mode(arg: &str) -> Result<String, String> {
+    if arg != "-" && arg != "librequake" && arg != "morrowind.exe" && arg != "openmw" {
+        return Err(format!("\"{}\" invalid compile mode", arg));
+    }
+    Ok(arg.into())
+}
+
 fn create_workdir(map_name: &String) -> (String, String) {
     let dir_index = map_name
         .rfind('/')