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/Misc/NEWS b/Misc/NEWS
index c74cb55..f9eb1cd 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -28,6 +28,9 @@
 Library
 -------
 
+- Issue #8286: The distutils command sdist will print a warning message instead
+  of crashing when an invalid path is given in the manifest template.
+
 - Issue #12636: IDLE reads the coding cookie when executing a Python script.
 
 - Issue #10946: The distutils commands bdist_dumb, bdist_wininst and bdist_msi