Update Warning-strings to use a standard-format

- ensure warnings always start with "Warning:" and that the first
  character after ":" is written uppercase
- ensure the first sentence in warnings ends with an "!"
- remove unneeded spaces from Warning-strings
- add missing Warning-string translation
- remove a "\n" and replace with untranslatable "<br><br>"

src/script.h

@@ -378,7 +378,7 @@ public:
     {
         // I'm not sure if this should push the script or concatenate scripts.
         // If there's ever a use for pushing a script onto a script, delete this member fn
-        assert(!"warning: pushing a CScript onto a CScript with << is probably not intended, use + to concatenate");
+        assert(!"Warning: Pushing a CScript onto a CScript with << is probably not intended, use + to concatenate!");
         return *this;
     }