Fix EvalScript by using verbatim string for #Load

This is in reference to fsharp/FAKE#501. This fix passes paths verbatim
so that paths such as C:\code\build.fsx do not fail due to the escape
sequence '\b' being present.
Honestly either would work as far as I'm concerned. I seriously doubt someone would create that file and run the regression test, so returning false is fine too. The nice thing is that regardless of the file's existence, the test would fail on regression.