setup.py: fix bad error message handling introduced by c2ac7c90

setup.py

@@ -208,21 +208,17 @@ class kwant_sdist(distutils_sdist):
 
         distutils_sdist.run(self)
 
-        msg = None
         if names is None:
-            msg = ['Git was not available for re-generating '
-                   'the MANIFEST file (the list of file',
-                   'names to be included in the source'
-                   ' distribution).  The old MANIFEST was used.']
-        if not trustworthy:
-            msg = ['-The existing MANIFEST file seems to'
-                   ' have been generated by distutils (it begins',
-                   '-with a comment).  It may well be incomplete.']
-
-        if msg is not None:
-            msg.insert(0, header(' Warning '))
-            print(msg, sep='\n', file=sys.stderr)
+            print(header(' Warning '),
+"""Git was not available for re-generating the MANIFEST file (the list of file
+names to be included in the source distribution).  The old MANIFEST was used.""",
+                  sep='\n', file=sys.stderr)
 
+        if not trustworthy:
+            print(header(' Warning '),
+"""The existing MANIFEST file seems to have been generated by distutils (it begins
+with a comment).  It may well be incomplete.""",
+                  sep='\n', file=sys.stderr)
 
 # Other than the "if not use_git" clause in the beginning, this is an exact copy
 # of the function from kwant/_common.py.  We can't import it here (because Kwant