state.File.set_static(): barf if we set_static() a nonexistent file.

If that ever happens, we probably got our paths mangled (like in the
previous commit) so we should die right away rather than allow weird things
to happen later.

state.py

@@ -216,7 +216,7 @@ class File(object):
         self.is_generated = True
 
     def set_static(self):
-        self.update_stamp()
+        self.update_stamp(must_exist=True)
         self.is_override = False
         self.is_generated = False
 
@@ -224,8 +224,10 @@ class File(object):
         self.update_stamp()
         self.is_override = True
 
-    def update_stamp(self):
+    def update_stamp(self, must_exist=False):
         newstamp = self.read_stamp()
+        if must_exist and newstamp == STAMP_MISSING:
+            raise Exception("%r does not exist" % self.name)
         if newstamp != self.stamp:
             debug2("STAMP: %s: %r -> %r\n" % (self.name, self.stamp, newstamp))
             self.stamp = newstamp