Tweak the OSX environment configuration script.

[in tools/buildbot/slaves/svn-x64-macosx]
* setenv.sh: Show the full ramdisk file path in the error message.
   Check that the ramdisk volume name is non-empty.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1638701 13f79535-47bb-0310-9956-ffa450edef68