Update lib/matplotlib/backend_bases.py

Co-authored-by: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

Author: ianthomas23

lib/matplotlib/backend_bases.py

@@ -108,7 +108,7 @@ def _safe_pyplot_import():
         from matplotlib.backends.registry import backend_registry
         backend = backend_registry.backend_for_gui_framework(current_framework)
         if backend is None:
-            raise KeyError(backend)
+            raise RuntimeError(f"No suitable backend for the current GUI framework {current_framework!r}")
 
         rcParams["backend"] = mpl.rcParamsOrig["backend"] = backend
         import matplotlib.pyplot as plt  # Now this should succeed.