Added warning if source file cannot be found

If source files are defined using relative paths, CMake will compile the tests, but this script will (sometimes) fail to find and parse the tests from the source files.  I have added an explicit warning when ParseAndAddCatchTests fails to find a source file.

contrib/ParseAndAddCatchTests.cmake

@@ -48,6 +48,7 @@ endfunction()
 # Worker function
 function(ParseFile SourceFile TestTarget)
     if(NOT EXISTS ${SourceFile})
+        message(WARNING "Cannot find source file: ${SourceFile}")
         return()
     endif()
     file(STRINGS ${SourceFile} Contents NEWLINE_CONSUME)
@@ -111,4 +112,4 @@ function(ParseAndAddCatchTests TestTarget)
     foreach(SourceFile ${SourceFiles})
         ParseFile(${SourceFile} ${TestTarget})
     endforeach()
-endfunction()
+endfunction()