fix: improve error messages and refactor large functions in save/diff modes

Co-authored-by: aider (openai/andrew/openrouter/qwen/qwen3-coder) <aider@aider.chat>

src/modes/common.rs

@@ -151,7 +151,7 @@ pub fn cmd_args_digest_type(cmd: &mut Command, args: &Args) -> MetaPluginType {
     if digest_type_opt.is_err() {
         cmd.error(
             ErrorKind::InvalidValue,
-            format!("Unknown digest type: {}", digest_name),
+            format!("Invalid digest algorithm '{}'. Use 'sha256' or 'md5'", digest_name),
         )
         .exit();
     }
@@ -170,7 +170,7 @@ pub fn cmd_args_compression_type(cmd: &mut Command, args: &Args) -> CompressionT
     if compression_type_opt.is_err() {
         cmd.error(
             ErrorKind::InvalidValue,
-            format!("Unknown compression type: {}", compression_name),
+            format!("Invalid compression algorithm '{}'. Supported algorithms: lz4, gzip, xz, zstd", compression_name),
         )
         .exit();
     }