remote/coordinator: fix ExporterError message

The message should have been an f-string to contain any useful
information.

Fixes: 8cb8ecd ("remote/coordinator: show error reason for failed exporter commands")
Signed-off-by: Bastian Krause <bst@pengutronix.de>
(cherry picked from commit 6560c35)

Author: Bastian-Krause

labgrid/remote/coordinator.py

@@ -626,7 +626,7 @@ async def _acquire_resource(self, place, resource):
         self.get_exporter_by_name(resource.path[0]).queue.put_nowait(cmd)
         await cmd.wait()
         if not cmd.response.success:
-            raise ExporterError("failed to acquire {resource} ({cmd.response.reason})")
+            raise ExporterError(f"failed to acquire {resource} ({cmd.response.reason})")
         if resource.acquired != place.name:
             logging.warning("resource %s not acquired by this place after acquire request", resource)