core: use debug format for logging errors

Show the full cause stack.

src/core/tile_merger.rs

@@ -68,7 +68,7 @@ pub trait TileMerger {
 				let timestamp = match fs::modified_timestamp(&source_path) {
 					Ok(timestamp) => timestamp,
 					Err(err) => {
-						warn!("{}", err);
+						warn!("{:?}", err);
 						return None;
 					}
 				};

src/core/tile_mipmapper.rs

@@ -127,7 +127,7 @@ where
 				Ok(source) => source,
 				Err(err) => {
 					warn!(
-						"Failed to read source image {}: {}",
+						"Failed to read source image {}: {:?}",
 						source_path.display(),
 						err,
 					);