Handle abort in exclusive output state correctly

If the event loop is stopped while something holds the exclusive output, the
"log" singleton is now reset so the main thread can print a few more messages
before exiting.

PFERD/logging.py

@@ -97,12 +97,28 @@ class Log:
                     self.print(line)
                 self._lines = []
 
+    def unlock(self) -> None:
+        """
+        Get rid of an exclusive output state.
+
+        This function is meant to let PFERD print log messages after the event
+        loop was forcibly stopped and if it will not be started up again. After
+        this is called, it is not safe to use any functions except the logging
+        functions (print, warn, ...).
+        """
+
+        self._progress_suspended = False
+        for line in self._lines:
+            self.print(line)
+
     def print(self, text: str) -> None:
         if self._progress_suspended:
             self._lines.append(text)
         else:
             self.console.print(text)
 
+    # TODO Print errors (and warnings?) to stderr
+
     def warn(self, text: str) -> None:
         self.print(f"[bold bright_red]Warning[/] {escape(text)}")