Fix the release_attachment file-existence loop

I only dealt with the scenario where I get back an Err(_) value. This is
not what happens when the file is missing, it's what happens when there
was some other problem interacting with the file. Instead, an Ok(bool)
is returned to indicate positive identification of the presence or
absence of the file.

Something should be done about the std::io::error::Error that gets
handed back. Since I'm not expecting to see it very often, nor to really
do anything about it, I'm just emitting an error message and panicking.

src/api/release_attachment.rs

@@ -16,8 +16,13 @@ pub async fn create_release_attachment(
 
     // Ensure all files exists before starting the uploads
     for file in &files {
-        if let Err(e) = fs::exists(file) {
-            return Err(crate::Error::NoSuchFile);
+        match fs::exists(file) {
+            Ok(true) => continue,
+            Ok(false) => return Err(crate::Error::NoSuchFile),
+            Err(e) => {
+                eprintln!("Uh oh! The file-exists check couldn't be done: {e}");
+                panic!("TODO: Deal with scenario where the file's existence cannot be checked (e.g.: no permission)");
+            },
         }
     }