Merge pull request matplotlib#4521 from jkseppan/issue-4475

ENH: Raise more useful error when tfm file is missing

Author: tacaswell

lib/matplotlib/dviread.py

@@ -395,15 +395,18 @@ def _xxx(self, special):
                 'debug')
 
     def _fnt_def(self, k, c, s, d, a, l, n):
-        tfm = _tfmfile(n[-l:].decode('ascii'))
+        fontname = n[-l:].decode('ascii')
+        tfm = _tfmfile(fontname)
+        if tfm is None:
+            raise FileNotFoundError("missing font metrics file: %s" % fontname)
         if c != 0 and tfm.checksum != 0 and c != tfm.checksum:
             raise ValueError('tfm checksum mismatch: %s'%n)
         # It seems that the assumption behind the following check is incorrect:
         #if d != tfm.design_size:
         #    raise ValueError, 'tfm design size mismatch: %d in dvi, %d in %s'%\
         #        (d, tfm.design_size, n)
 
-        vf = _vffile(n[-l:].decode('ascii'))
+        vf = _vffile(fontname)
 
         self.fonts[k] = DviFont(scale=s, tfm=tfm, texname=n, vf=vf)