more verbose message when dealing with symlinks

@ussuri, added a message when dealing  with symlinks.


Can we confidently say that this condition here does and always will indicate the problem with folders containing symlinks? I think what should be done instead is:
* define new error codes in SparkErrorConstants, similar to RUN_APP_NOT_FOUND_IN_CHROME (and while at it, do the same for PUB_ON_WINDOWS_MSG, to maintain uniformity);
* intercept FileError much closer to the source, where it unambiguously indicate the symlink problem;
* rethrow a SparkException with the mentioned new error code and message from there; 
* remove the special handling here.