[ruby-core:73931] [Ruby trunk Feature#12101] Add verbose failure messages and avoid infamous DRb::DRbConnError

Issue #12101 has been updated by Michael Yagudaev.

Tracker changed from Bug to Feature

----------------------------------------
Feature #12101: Add verbose failure messages and avoid infamous DRb::DRbConnError
https://bugs.ruby-lang.org/issues/12101#change-57082

* Author: Michael Yagudaev
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
Avoid the generic "DRb::DRbConnError" which gives very little information and make errors hard to debug.

When I stared using DRuby, I kept getting a strange and cryptic error message `DRb::DRbConnError 'connection closed'`. Turns out it was due to some silly mistake on the server side of the relationship. It took hours to figure out. The only way I was able to do it was to patch ruby drb after closely studying the source.

Others have had the same issues:

http://stackoverflow.com/questions/27293817/ruby-connection-closed-drbdrbconnerror
https://www.ruby-forum.com/topic/193984
https://github.com/Mon-Ouie/pry-remote/issues/8

Here is an example where this makes a difference: 

```ruby
# drb_server.rb
require 'drb'

class FrontObject
  def foo
    # generates Runtime Exception of Insecure Operation
    result = `ls -l`
  end
end

DRb.start_service("druby://localhost:8787", FrontObject.new, safe_level: 3)
DRb.thread.join
```

``` ruby
# drb_client.rb
require 'drb'

object = DRbObject.new_with_uri("druby://localhost:8787")
puts object.foo
```

Output before patch is applied:

```
$ ruby drb_server.rb
```

```
$ ruby drb_client.rb
/Users/mike/.rbenv/versions/2.2.3/lib/ruby/2.2.0/drb/drb.rb:578:in `load': connection closed (DRb::DRbConnError)
```

Output after patch is applied:

```
$ ruby drb_server.rb
Insecure operation ``' at level 3
```

```
$ ruby drb_client.rb
/Users/mike/.rbenv/versions/2.2.3/lib/ruby/2.2.0/drb/drb.rb:578:in `load': connection closed (DRb::DRbConnError)
```

Original issue and proposed solution started on github: https://github.com/ruby/ruby/pull/1260



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