Warn if MPLBACKEND is invalid.

anntzer · anntzer · commit cfb2d3dd01f7 · 2016-07-06T10:56:14.000-07:00
I could be convinced to make this an error actually.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -1460,7 +1460,9 @@ def tk_window_focus():
     # variable MPLBACKEND
     try:
         use(os.environ['MPLBACKEND'])
-    except (KeyError, ValueError):
+    except ValueError as e:
+        warnings.warn(str(e))  # Invalid backend.
+    except KeyError:
         pass
 
 
