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.

Meta/CMake/utils.cmake

@@ -98,7 +98,7 @@ function(serenity_app target_name)
     if (EXISTS "${small_icon}")
         embed_resource("${target_name}" serenity_icon_s "${small_icon}")
     else()
-        message(WARNING "Missing small app icon: ${small_icon}")
+        message(FATAL_ERROR "Missing small app icon: ${small_icon}")
     endif()
 
     if (EXISTS "${medium_icon}")