android(toolchain): detailed error message

platforms/android/android.toolchain.cmake

@@ -810,7 +810,7 @@ unset(__real_api_level)
 # validate
 list( FIND ANDROID_SUPPORTED_NATIVE_API_LEVELS "${ANDROID_NATIVE_API_LEVEL}" __levelIdx )
 if( __levelIdx EQUAL -1 )
- message( SEND_ERROR "Specified Android native API level 'android-${ANDROID_NATIVE_API_LEVEL}' is not supported by your NDK/toolchain." )
+ message( SEND_ERROR "Specified Android native API level 'android-${ANDROID_NATIVE_API_LEVEL}' is not supported by your NDK/toolchain.\nSupported values of ANDROID_NATIVE_API_LEVEL: ${ANDROID_SUPPORTED_NATIVE_API_LEVELS}" )
 else()
  if( BUILD_WITH_ANDROID_NDK )
   __DETECT_NATIVE_API_LEVEL( __realApiLevel "${ANDROID_NDK}/platforms/android-${ANDROID_NATIVE_API_LEVEL}/arch-${ANDROID_ARCH_NAME}/usr/include/android/api-level.h" )