[Build] Use FATAL_ERROR, not ERROR.

We should be using `FATAL_ERROR` for error messages from CMake, not
just `ERROR`.

Author: al45tair

Runtimes/Core/cmake/modules/PlatformInfo.cmake

@@ -50,26 +50,26 @@ if(NOT SwiftCore_SWIFT_AVAILABILITY_PLATFORM)
   elseif(SwiftCore_MODULE_TRIPLE MATCHES ".*-([^-]+)$")
     set(platform "${CMAKE_MATCH_1}")
   else()
-    message(ERROR "Unable to extract platform name from triple ${SwiftCore_MODULE_TRIPLE}")
+    message(FATAL_ERROR "Unable to extract platform name from triple ${SwiftCore_MODULE_TRIPLE}")
   endif()
 
   if(availability_platform_${platform})
     set(SwiftCore_SWIFT_AVAILABILITY_PLATFORM "${availability_platform_${platform}}")
   else()
-    message(ERROR "Unknown platform ${platform} for availability")
+    message(FATAL_ERROR "Unknown platform ${platform} for availability")
   endif()
 endif()
 
 if(SwiftCore_VARIANT_MODULE_TRIPLE)
   if(SwiftCore_VARIANT_MODULE_TRIPLE MATCHES ".*-([^-]+)$")
     set(platform "${CMAKE_MATCH_1}")
   else()
-    message(ERROR "Unable to extract platform name from triple ${SwiftCore_VARIANT_MODULE_TRIPLE}")
+    message(FATAL_ERROR "Unable to extract platform name from triple ${SwiftCore_VARIANT_MODULE_TRIPLE}")
   endif()
 
   if(availability_platform_${platform})
     set(SwiftCore_VARIANT_AVAILABILITY_PLATFORM "${availability_platform_${platform}}")
   else()
-    message(ERROR "Unknown platform ${platform} for variant availability")
+    message(FATAL_ERROR "Unknown platform ${platform} for variant availability")
   endif()
 endif()