If the original file is missing, make it non-fatal

The external data code makes it hard to add a new test as the test
is used to generate the baseline. Make the missing original a status
message rather than a fatal error. This allows you to add a new test,
compile it, generate the baseline and then add it as we did before the
move the external data.

Change-Id: I7cf25270c59ec1055c9e4c37d9e328f9f696f236

CMake/ExternalData.cmake

@@ -449,7 +449,7 @@ function(_ExternalData_arg target arg options var_file)
   endif()
 
   if(NOT have_original)
-    message(FATAL_ERROR "Data file referenced by argument\n"
+    message(STATUS "Data file referenced by argument\n"
       "  ${arg}\n"
       "corresponds to source tree path\n"
       "  ${reldata}\n"