Use colour to make errors more obvious

src/main.rs

@@ -59,6 +59,7 @@ mod words;
 #[macro_use]
 extern crate log;
 
+use crossterm::tty::IsTty as _;
 use display::style::print_warning;
 use log::info;
 use options::FilePermissions;
@@ -518,7 +519,12 @@ fn diff_conflicts_file(
 
     if conflict_files.num_conflicts == 0 {
         eprintln!(
-            "error: Difftastic requires two paths, or a single file with conflict markers {}.\n",
+            "{}: Difftastic requires two paths, or a single file with conflict markers {}.\n",
+            if std::io::stdout().is_tty() {
+                "error".red().bold().to_string()
+            } else {
+                "error".to_owned()
+            },
             START_LHS_MARKER,
         );
 

src/options.rs

@@ -958,7 +958,12 @@ pub(crate) fn parse_args() -> Mode {
         _ => {
             if !args.is_empty() {
                 eprintln!(
-                    "error: Difftastic does not support being called with {} argument{}.\n",
+                    "{}: Difftastic does not support being called with {} argument{}.\n",
+                    if use_color {
+                        "error".red().bold().to_string()
+                    } else {
+                        "error".to_owned()
+                    },
                     args.len(),
                     if args.len() == 1 { "" } else { "s" }
                 );