RejectedExceptions should include the configuration keys that users can tune the thresholds with

[robhruska]

Here's a rejected exception from Mjolnir's current version:

2015-11-19 16:13:54,871 ip-0AB43960 [WARN ] [Hudl.Multiverse.Web.Mvc.Attributes.Usage] [209] [request_id=cf5d9a05] [requested_branch=(null)] Error writing usage log AttemptedFields=Hudl|UnFollow|Topic
Type=System.AggregateException
Message=One or more errors occurred.
Data:
    Signature=e017ce6d
Stack Trace: <empty>
==== Inner Exception ====
Type=Hudl.Mjolnir.Command.CommandFailedException
Message=Command rejected
Data:
    Command=users.IUserService-GetUser
    Timeout=15000
    Status=Rejected
    Breaker=user
    Pool=user
Stack Trace:
   at Hudl.Mjolnir.Command.Command`1.Fallback(CommandFailedException instigator) in z:\work\f128c7cb4a95d90e\Hudl.Mjolnir\Command\Command.cs:line 503
   at Hudl.Mjolnir.Command.Command`1.TryFallback(CommandFailedException instigator) in z:\work\f128c7cb4a95d90e\Hudl.Mjolnir\Command\Command.cs:line 466
   at Hudl.Mjolnir.Command.Command`1.<InvokeAsync>d__9.MoveNext() in z:\work\f128c7cb4a95d90e\Hudl.Mjolnir\Command\Command.cs:line 320
==== Inner Exception ====
Type=Hudl.Mjolnir.ThreadPool.IsolationThreadPoolRejectedException
Message=Exception of type 'Hudl.Mjolnir.ThreadPool.IsolationThreadPoolRejectedException' was thrown.
Data:
Stack Trace:
   at Hudl.Mjolnir.ThreadPool.StpIsolationThreadPool.Enqueue[TResult](Func`1 func) in z:\work\f128c7cb4a95d90e\Hudl.Mjolnir\ThreadPool\StpIsolationThreadPool.cs:line 107
   at Hudl.Mjolnir.Command.Command`1.ExecuteInIsolation(CancellationToken cancellationToken) in z:\work\f128c7cb4a95d90e\Hudl.Mjolnir\Command\Command.cs:line 344
   at Hudl.Mjolnir.Command.Command`1.<InvokeAsync>d__9.MoveNext() in z:\work\f128c7cb4a95d90e\Hudl.Mjolnir\Command\Command.cs:line 293

The message (Command rejected) is pretty terse. It also means that users have to go dig through documentation to figure out how to re-tune the thread pool.

A clearer message might be something like:

Command was rejected because it exceeded a thread pool (concurrency) limit of 10. To adjust this value, set the mjolnir.pools.user.threadCount configuration value.

Look for contextual opportunities to explain why an exception happened and how a user can change things to fix the problem.