Unhelpful error message if shaidy server is read only.

[marohrdanz]

If a user is configured to use a read-only shaidy server (via chmCreateServer) and tries to create a collection with

chmCreateCollection("//demo/mynewcollection/")

then the code crashes with unhelpful error message:

Error in validObject(.Object) : 
  invalid class “shaid” object: invalid object for slot "value" in class "shaid": got class "NULL", should be or extend class "character"

Desired behavior: Provide more meaningful feedback to the user

NOTE: we currently don't advertise shaidy server functions to general users. Hence the 'low-priority' tag.