font-patcher: Fix behavior when trying to patch non-font files

[why]
When the file specified to be patched is not a font file the patcher run
errors out with an out of index runtime error:

Traceback (most recent call last):
  File "/home/fini/extra/git/nerd-fonts/font-patcher", line 2155, in <module>
    main()
  File "/home/fini/extra/git/nerd-fonts/font-patcher", line 2147, in main
    patcher.generate(sourceFonts)
  File "/home/fini/extra/git/nerd-fonts/font-patcher", line 415, in generate
    sourceFont = sourceFonts[0]
                 ~~~~~~~~~~~^^^
IndexError: list index out of range

[how]
Do not assume that the specified file will be a font file but rather
check if fontforge detects a font in the file and error out if there is
no font found.

Fixes: #1647

Reported-by: Kristopher James Kent <[email protected]>
Signed-off-by: Fini Jastrow <[email protected]>

font-patcher

@@ -2127,6 +2127,9 @@ def main():
 
     sourceFonts = []
     all_fonts = fontforge.fontsInFile(args.font)
+    if not all_fonts:
+        logger.critical("Can not find any fonts in '%s'", args.font)
+        sys.exit(1)
     for i, subfont in enumerate(all_fonts):
         if len(all_fonts) > 1:
           print("\n")