refactor: migrate panic!() usage to Rust 2021

This commit refactors code to conform to some changes in Rust 2021:
- The panic!() macro now uses format_args!() like println!().
- The usage of panic!(x) is now deprecated if x is not a string literal.

See:
https://doc.rust-lang.org/nightly/edition-guide/rust-2021/panic-macro-consistency.html

src/components/editor/mod.rs

@@ -67,7 +67,7 @@ impl EchidnaCoreEditor {
                     |_, _| {},
                     |result| {
                         if result.is_err() {
-                            panic!(result.err());
+                            panic!("Found an error when loading the file into the text editor's buffer. {:#?}", result.err());
                         }
                     },
                 );

src/components/window/file.rs

@@ -114,7 +114,7 @@ impl FileImplementedEditor for super::EchidnaWindow {
                         "Couldn't get the page of the current index. Try again."
                     ).downcast::<EchidnaCoreEditor>() {
                     Ok(res) => page = res,
-                    Err(e) => panic!(format!("We got an error when trying to downcast the current tab page into EchidnaCoreEditor:\n{}", e))
+                    Err(e) => panic!("We got an error when trying to downcast the current tab page into EchidnaCoreEditor:\n{}", e)
                 }
 
                 let buffer: Buffer = page.to_imp().sourceview.buffer().downcast().expect("Could not downcast the editor's buffer to GtkSourceBuffer.");
@@ -129,7 +129,7 @@ impl FileImplementedEditor for super::EchidnaWindow {
                     |_, _| {},
                     |result| {
                         if result.is_err() {
-                            panic!(format!("Found an error while saving the file:\n{}", result.err().expect("No error")))
+                            panic!("Found an error while saving the file:\n{}", result.err().expect("No error"))
                         }
                     });