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.")

Kernel/CMakeLists.txt

@@ -353,7 +353,7 @@ if (${CMAKE_HOST_SYSTEM_NAME} MATCHES SerenityOS)
  include_directories(/usr/local/include/c++/${GCC_VERSION}/)
 else()
  if (NOT EXISTS ${TOOLCHAIN_ROOT}/Kernel/${SERENITY_ARCH}-pc-serenity/include/c++/${GCC_VERSION}/)
- message(SEND_ERROR "Toolchain version ${GCC_VERSION} appears to be missing! Please run: Meta/serenity.sh rebuild-toolchain")
+ message(FATAL_ERROR "Toolchain version ${GCC_VERSION} appears to be missing! Please run: Meta/serenity.sh rebuild-toolchain")
  endif()
  include_directories(${TOOLCHAIN_ROOT}/Kernel/${SERENITY_ARCH}-pc-serenity/include/c++/${GCC_VERSION}/)
  include_directories(${TOOLCHAIN_ROOT}/Kernel/${SERENITY_ARCH}-pc-serenity/include/c++/${GCC_VERSION}/${SERENITY_ARCH}-pc-serenity/)

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}")