In findfont(fallback_to_default=False), throw if default font is missing.

... instead of entering an infinite loop of findfont -> defaultFont ->
findfont.

A bit of a pain to test (one needs to delete dejavusans both from the
mpl source tree and from the system font folder)...

This should only affect people who delete the default fonts anyways (or
call with fallback_to_default=False), so I'll skip the API change note.

Author: anntzer

lib/matplotlib/font_manager.py

@@ -1284,11 +1284,8 @@ def _findfont_cached(self, prop, fontext, directory, fallback_to_default,
                 default_prop.set_family(self.defaultFamily[fontext])
                 return self.findfont(default_prop, fontext, directory, False)
             else:
-                # This is a hard fail -- we can't find anything reasonable,
-                # so just return the DejaVuSans.ttf
-                _log.warning('findfont: Could not match %s. Returning %s.',
-                             prop, self.defaultFont[fontext])
-                result = self.defaultFont[fontext]
+                raise ValueError(f"Failed to find font {prop}, and fallback "
+                                 f"to the default font was disabled")
         else:
             _log.debug('findfont: Matching %s to %s (%r) with score of %f.',
                        prop, best_font.name, best_font.fname, best_score)