Notify user when running in an encrypted collaboration without private key

[mellesies]

Currently the error message doesn't give any hints that (failed) encryption might be the reason:

> vtg.basic::colnames(client)
  waiting for results for task '/task/132' in  8s
11:02:02 - ERROR (value[[3L]]): could not read results:
11:02:02 - ERROR (value[[3L]]): Site results:
11:02:02 - ERROR (value[[3L]]): 
Error in marshalled.result[["result"]] : subscript out of bounds
In addition: Warning messages:
1: [2020-05-25 11:02:02.647] An error occurred during logging: Error in parse(text = text, keep.source = FALSE): <text>:2:1625: unexpected ','
2: UXZ5qWJDLIeS3Zdb134SnZdYIMn1rrf20zBTnF7+W+8rtA9Sn4Ygre3ZgB2zfRMwu6SSDXG/rMFwdVdbuYlE5ZeSzrxrNLQgMeYlLFGLXN6tB3ha3jwoMnIEJvDeUeSnZ5HlZ1dkjmw+c09M9i72WdK4QCNvrv0xp39RQ/zXExQQbUSPQBcGVfIXL80cTAZn
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         [... truncated] 
 Error in marshalled.result[["result"]] : subscript out of bounds 
3. self$process.results(site_results) 
2. client$call("colnames") at colnames.R#17
1. vtg.basic::colnames(client)