improve "no image loaders" error message

crates/egui/src/context.rs

@@ -2038,17 +2038,25 @@ impl Context {
     ///
     /// # Errors
     /// This may fail with:
+    /// - [`LoadError::NoImageLoaders`][no_image_loaders] if tbere are no registered image loaders.
     /// - [`LoadError::NotSupported`][not_supported] if none of the registered loaders support loading the given `uri`.
     /// - [`LoadError::Custom`][custom] if one of the loaders _does_ support loading the `uri`, but the loading process failed.
     ///
     /// ⚠ May deadlock if called from within an `ImageLoader`!
     ///
+    /// [no_image_loaders]: crate::load::LoadError::NoImageLoaders
     /// [not_supported]: crate::load::LoadError::NotSupported
     /// [custom]: crate::load::LoadError::Custom
     pub fn try_load_image(&self, uri: &str, size_hint: load::SizeHint) -> load::ImageLoadResult {
         crate::profile_function!();
 
-        for loader in self.loaders().image.lock().iter() {
+        let loaders = self.loaders();
+        let loaders = loaders.image.lock();
+        if loaders.is_empty() {
+            return Err(load::LoadError::NoImageLoaders);
+        }
+
+        for loader in loaders.iter() {
             match loader.load(self, uri, size_hint) {
                 Err(load::LoadError::NotSupported) => continue,
                 result => return result,

crates/egui/src/load.rs

@@ -66,6 +66,9 @@ use std::{error::Error as StdError, fmt::Display, sync::Arc};
 /// Represents a failed attempt at loading an image.
 #[derive(Clone, Debug)]
 pub enum LoadError {
+    /// There are no image loaders installed.
+    NoImageLoaders,
+
     /// This loader does not support this protocol or image format.
     NotSupported,
 
@@ -76,6 +79,7 @@ pub enum LoadError {
 impl Display for LoadError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
+            LoadError::NoImageLoaders => f.write_str("no image loaders are installed"),
             LoadError::NotSupported => f.write_str("not supported"),
             LoadError::Custom(message) => f.write_str(message),
         }