Report an error when computecpp_info or compute++ can't be found (#126)

Author: RossBrunton

cmake/Modules/FindComputeCpp.cmake

@@ -79,10 +79,16 @@ endif()
 find_program(ComputeCpp_DEVICE_COMPILER_EXECUTABLE compute++
  HINTS ${computecpp_host_find_hint}
  PATH_SUFFIXES bin)
+if(NOT ComputeCpp_DEVICE_COMPILER_EXECUTABLE) 
+ message(FATAL_ERROR "Could not find compute++.")
+endif()
 
 find_program(ComputeCpp_INFO_EXECUTABLE computecpp_info
  HINTS ${computecpp_host_find_hint}
  PATH_SUFFIXES bin)
+if(NOT ComputeCpp_INFO_EXECUTABLE) 
+ message(FATAL_ERROR "Could not find computecpp_info.")
+endif()
 
 find_library(COMPUTECPP_RUNTIME_LIBRARY
  NAMES ComputeCpp ComputeCpp_vs2015