CMakeLists: Enforce C4715 on MSVC

This is similar to -Werror=return-type

src/CMakeLists.txt

@@ -54,6 +54,7 @@ if (MSVC)
         /we4547 # 'operator' : operator before comma has no effect; expected operator with side-effect
         /we4549 # 'operator1': operator before comma has no effect; did you intend 'operator2'?
         /we4555 # Expression has no effect; expected expression with side-effect
+        /we4715 # 'function': not all control paths return a value
         /we4834 # Discarding return value of function with 'nodiscard' attribute
         /we5038 # data member 'member1' will be initialized after data member 'member2'
     )