rest: improve error message and status code for symlinks

[audrium]

After introducing rest: avoid following symlinks inside workspaces we now raise a general exception in case someone tries to do any file operation to a symlink:

rc download -w test symlinkfile
==> ERROR: File symlinkfile could not be downloaded: Error 500 Internal Server Error Symlinks not allowed

as an MVP we could improve the error message and status code to handle this case gratefully.

Alternatively, we could let user know that the manipulated file is a symlink. The server would have had to send this "file type" information along with the reply.

For example even for successful messages:

$ reana-client rm -w rucio etc
==> SUCCESS: File etc was successfully deleted.
==> SUCCESS: 4 bytes freed up.

Here etc is a symlink, but client does not know that. We could improve the response from the client to inform the user accordingly.