Add message for missing submodule if websocketspp is missing. (microsoft#930)

Author: BillyONeal

Release/cmake/cpprest_find_websocketpp.cmake

@@ -7,9 +7,11 @@ function(cpprest_find_websocketpp)
   if(WEBSOCKETPP_FOUND)
     message("-- Found websocketpp version " ${WEBSOCKETPP_VERSION} " on system")
     set(WEBSOCKETPP_INCLUDE_DIR ${WEBSOCKETPP_INCLUDE_DIR} CACHE INTERNAL "")
-  else()
+  elseif(EXISTS ${PROJECT_SOURCE_DIR}/libs/websocketpp/CMakeLists.txt)
     message("-- websocketpp not found, using the embedded version")
     set(WEBSOCKETPP_INCLUDE_DIR ${PROJECT_SOURCE_DIR}/libs/websocketpp CACHE INTERNAL "")
+  else()
+    message(FATAL_ERROR "-- websocketpp not found and embedded version not present; try `git submodule update --init` and run CMake again")
   endif()
 
   cpprest_find_boost()
@@ -22,4 +24,4 @@ function(cpprest_find_websocketpp)
       cpprestsdk_boost_internal
       cpprestsdk_openssl_internal
   )
-endfunction()
+endfunction()