Don't print an empty "error:" line in the CLI

This change in behavior was introduced in tectonic-typesetting#636 with the genericization
of the status backend. Add a new `report_error` method to recover the
old behavior.

Closes tectonic-typesetting#665.

src/bin/tectonic.rs

@@ -261,12 +261,12 @@ fn main() {
         "this is a BETA release; ask questions and report bugs at https://tectonic.newton.cx/"
     );
 
-    // Now that we've got colorized output, we're to pass off to the inner
-    // function ... all so that we can print out the word "error:" in red.
-    // This code parallels various bits of the `error_chain` crate.
+    // Now that we've got colorized output, pass off to the inner function ...
+    // all so that we can print out the word "error:" in red. This code
+    // parallels various bits of the `error_chain` crate.
 
     if let Err(ref e) = inner(args, config, &mut *status) {
-        tt_error!(status, ""; e);
+        status.report_error(e);
         process::exit(1)
     }
 }

src/status/mod.rs

@@ -63,8 +63,25 @@ pub enum MessageKind {
 
 pub trait StatusBackend {
     /// Report a message to the status backend.
+    ///
+    /// If `err` is not None, it represents an error that somehow caused the
+    /// current message to be reported. It should be displayed in some
+    /// appropriate fashion.
     fn report(&mut self, kind: MessageKind, args: Arguments, err: Option<&Error>);
 
+    /// Report an error to the status backend.
+    ///
+    /// Unlike the basic `report` function, in this case there is no additional
+    /// contextual information provided. The default implementation delegates to
+    /// `report()` with a generic lead-in message of "an error occurred".
+    fn report_error(&mut self, err: &Error) {
+        self.report(
+            MessageKind::Error,
+            format_args!("an error occurred"),
+            Some(err),
+        )
+    }
+
     /// Issue a note-level status, idealy highlighting a particular phrase.
     ///
     /// This is a bit of a hack. For [`driver::ProcessingSession::run`], I

src/status/plain.rs

@@ -41,6 +41,20 @@ impl StatusBackend for PlainStatusBackend {
         }
     }
 
+    fn report_error(&mut self, err: &Error) {
+        let mut prefix = "error";
+
+        for item in err.iter() {
+            eprintln!("{}: {}", prefix, item);
+            prefix = "caused by";
+        }
+
+        if let Some(backtrace) = err.backtrace() {
+            eprintln!("debugging: backtrace follows:");
+            eprintln!("{:?}", backtrace);
+        }
+    }
+
     fn note_highlighted(&mut self, before: &str, highlighted: &str, after: &str) {
         if self.chatter > ChatterLevel::Minimal {
             self.report(

src/status/termcolor.rs

@@ -168,6 +168,27 @@ impl StatusBackend for TermcolorStatusBackend {
         }
     }
 
+    fn report_error(&mut self, err: &Error) {
+        let mut first = true;
+        let kind = MessageKind::Error;
+
+        for item in err.iter() {
+            if first {
+                self.generic_message(kind, None, format_args!("{}", item));
+                first = false;
+            } else {
+                self.generic_message(kind, Some("caused by:"), format_args!("{}", item));
+            }
+        }
+
+        if let Some(backtrace) = err.backtrace() {
+            self.generic_message(kind, Some("debugging:"), format_args!("backtrace follows:"));
+            self.with_stream(kind, |s| {
+                writeln!(s, "{:?}", backtrace).expect("backtrace dump failed");
+            });
+        }
+    }
+
     fn note_highlighted(&mut self, before: &str, highlighted: &str, after: &str) {
         if self.chatter > ChatterLevel::Minimal {
             write!(self.stdout, "{}", before).expect("write to stdout failed");