[PATCH] Some checks added to assert existence of files.

[Noorul Islam]

Hello all,

Attached is a small patch which was useful while setting up my environment.

Summary:

* Uses file-name-as-directory function to avoid mistakes that user
make while setting wave-client-root-dir.
* Checks whether the java program path is correct, if not errors out.
* Checks whether wave-client-root-dir has the script start-repl.sh, if
not errors out.

Thanks and Regards
Noorul