Handle errors better in the progress manager

Now the progress display will recognize errors using the `__exit__`
magic method and print out an error header with the actual error
message. This is useful because when the mouse leaves the Blender
operator's report error, the error "window" is destroyed. This led to
potential silent failures.

korman/exporter/logger.py

@@ -97,6 +97,18 @@ class ExportProgressLogger(_ExportLogger):
         self._time_start_overall = 0
         self._time_start_step = 0
 
+    def __exit__(self, type, value, traceback):
+        if value is not None:
+            export_time = time.perf_counter() - self._time_start_overall
+            with self._print_condition:
+                self._progress_print_step(done=(self._step_progress == self._step_max), error=True)
+                self._progress_print_line("\nABORTED AFTER {:.2f}s".format(export_time))
+                self._progress_print_heading("ERROR")
+                self._progress_print_line(str(value))
+                self._progress_print_heading()
+                self._progress_alive = False
+        return super().__exit__(type, value, traceback)
+
     def progress_add_step(self, name):
         assert self._step_id == -1
         self._progress_steps.append(name)
@@ -164,18 +176,18 @@ class ExportProgressLogger(_ExportLogger):
         else:
             self._progress_print_line("-" * _HEADING_SIZE)
 
-    def _progress_print_step(self, done=False):
+    def _progress_print_step(self, done=False, error=False):
         with self._print_condition:
             if done:
                 stage = "DONE IN {:.2f}s".format(time.perf_counter() - self._time_start_step)
                 print_func = self._progress_print_line
                 self._progress_print_volatile("")
             else:
-                if self._step_max != 0:
+                if self._step_max != 0 and self._step_progress != 0:
                     stage = "{} of {}".format(self._step_progress, self._step_max)
                 else:
                     stage = ""
-                print_func = self._progress_print_volatile
+                print_func = self._progress_print_line if error else self._progress_print_volatile
 
             line = "{}\t(step {}/{}): {}".format(self._progress_steps[self._step_id], self._step_id+1,
                                                 len(self._progress_steps), stage)