Surface the real failure cause in the error dialog (roadmap A3)

On failure the error window showed a generic message with the whole log
buried in an expander. Lead instead with a focused summary: which step
failed (the process label), its command, and the last stderr lines, while
keeping the full log below. The runner passes the failing process to
error_window, which reads the executor's bounded stderr_data.

build_summary/_stderr_tail are static and unit-tested (tests/
test_error_summary.py): tail extraction, CR handling, blank-line skipping,
stdout fallback, long-line truncation, and Pango-markup escaping of paths/
messages containing & < ' so set_markup can't choke.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/devedeng/error.py

@@ -14,14 +14,14 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>
 
-from gi.repository import Gtk, Gdk
+from gi.repository import Gtk, Gdk, GLib
 import os
 import devedeng.configuration_data
 
 
 class error_window:
 
-    def __init__(self):
+    def __init__(self, process=None):
 
         self.config = devedeng.configuration_data.configuration.get_config()
 
@@ -34,10 +34,60 @@ class error_window:
         wdebug_buffer = builder.get_object("debug_buffer")
         wdebug_buffer.set_text(self.config.get_log())
 
+        # lead with a focused summary of what actually failed, so the cause is
+        # visible without digging through the full log in the expander
+        wlabel = builder.get_object("label1")
+        if (wlabel is not None) and (process is not None):
+            wlabel.set_line_wrap(True)
+            wlabel.set_max_width_chars(80)
+            wlabel.set_selectable(True)
+            wlabel.set_markup(error_window.build_summary(process))
+
         werror_window.show_all()
         werror_window.run()
         werror_window.destroy()
 
+    @staticmethod
+    def build_summary(process):
+        esc = GLib.markup_escape_text
+        parts = ["<b>" + esc(_("The disc could not be created.")) + "</b>"]
+
+        step = getattr(process, "text", "") or ""
+        if step:
+            parts.append("")
+            parts.append(esc(_("Failed step: %s") % step))
+
+        command = getattr(process, "command_var", None)
+        if command:
+            cmd_str = " ".join(str(c) for c in command)
+            if len(cmd_str) > 400:
+                cmd_str = cmd_str[:400] + " ..."
+            parts.append(esc(_("Command: %s") % cmd_str))
+
+        tail = error_window._stderr_tail(process)
+        if tail:
+            parts.append("")
+            parts.append(esc(_("Last messages:")))
+            parts.append("<tt>" + esc(tail) + "</tt>")
+
+        parts.append("")
+        parts.append(esc(_("See the full log below for details.")))
+        return "\n".join(parts)
+
+    @staticmethod
+    def _stderr_tail(process, max_lines=12, max_line_len=200):
+        raw = getattr(process, "stderr_data", "") or ""
+        if not raw:
+            raw = getattr(process, "stdout_data", "") or ""
+        if not raw:
+            return ""
+        lines = [l.strip() for l in raw.replace("\r", "\n").split("\n")]
+        lines = [l for l in lines if l]
+        tail = lines[-max_lines:]
+        tail = [(l[:max_line_len] + " ...") if len(l) > max_line_len else l
+                for l in tail]
+        return "\n".join(tail)
+
     def on_copy_clicked(self, b):
 
         clipboard = Gtk.Clipboard.get(Gdk.SELECTION_CLIPBOARD)

src/devedeng/runner.py

@@ -132,7 +132,7 @@ class runner(GObject.GObject):
             for element in self.proc_list:
                 element.cancel()
             self.wprogress.destroy()
-            devedeng.error.error_window()
+            devedeng.error.error_window(process)
             self.emit("done", 1)  # there was an error
             return