Warn instead of crashing because of invalid path in MANIFEST.in (#8286).

sdist used to crash with a full traceback dump instead of printing a
nice warning with the faulty line number.
diff --git a/Lib/distutils/command/sdist.py b/Lib/distutils/command/sdist.py
index 21ea61d..a9429a4 100644
--- a/Lib/distutils/command/sdist.py
+++ b/Lib/distutils/command/sdist.py
@@ -306,7 +306,10 @@
 
                 try:
                     self.filelist.process_template_line(line)
-                except DistutilsTemplateError as msg:
+                # the call above can raise a DistutilsTemplateError for
+                # malformed lines, or a ValueError from the lower-level
+                # convert_path function
+                except (DistutilsTemplateError, ValueError) as msg:
                     self.warn("%s, line %d: %s" % (template.filename,
                                                    template.current_line,
                                                    msg))