CMake: Fix message levels for error conditions during configuration

Make messages which should be fatal, actually fail the build.

- FATAL is not a valid mode keyword. The full list is available in the
  docs: https://cmake.org/cmake/help/v3.19/command/message.html

- SEND_ERROR doesn't immediately stop processing, FATAL_ERROR does.
  We should immediately stop if the Toolchain is not present.

- The app icon size validation was just a WARNING that is easy to
  overlook. We should promote it to a FATAL_ERROR so that people will
  not overlook the issue when adding a new application. We can only make
  the small icon message FATAL_ERROR, as there is currently one
  violation of the medium app icon validation.

CMakeLists.txt

@@ -2,7 +2,7 @@ cmake_minimum_required(VERSION 3.16)
 project(SerenityOS C CXX ASM)
 
 if(NOT "${CMAKE_BUILD_TYPE}" STREQUAL "")
-  message(FATAL
+  message(FATAL_ERROR
     ": Don't use CMAKE_BUILD_TYPE when building serenity.\n"
     "The default build type is optimized with debug info and asserts enabled,\n"
     "and that's all there is.")