[usability] more helpful missing config message

[bryanhuntesl]

In the case that the config is missing the crash report/message is terrifying for new users.

It might be worthwhile to add a pre-flight check and print something more like :

libcluster -> missing config x 

21:54:49.998 [error] CRASH REPORT Process <0.374.0> with 0 neighbours crashed with reason: #{'__exception__' => true,'__struct__' => 'Elixir.Protocol.UndefinedError',description 
=> <<>>,protocol => 'Elixir.Enumerable',value => nil} in 'Elixir.Enumerable':'impl_for!'/1 line 1                                                                                 
21:54:49.998 [error] Supervisor noslides_sup had child 'Elixir.Cluster.Supervisor' started with 'Elixir.Cluster.Supervisor':start_link([nil,[{name,'Elixir.NoSlides.ClusterSuperviso
r'}]]) at undefined exit with reason #{'__exception__' => true,'__struct__' => 'Elixir.Protocol.UndefinedError',description => <<>>,protocol => 'Elixir.Enumerable',value => nil} 
in 'Elixir.Enumerable':'impl_for!'/1 line 1 in context start_error                                                                                                                
                                                                                                                                                                                  
21:54:50.014 [error] Unable to start NoSlides supervisor because: {:shutdown, {:failed_to_start_child, Cluster.Supervisor, {%Protocol.UndefinedError{description: "", protocol: Enu
merable, value: nil}, [{Enumerable, :impl_for!, 1, [file: 'lib/enum.ex', line: 1]}, {Enumerable, :reduce, 3, [file: 'lib/enum.ex', line: 141]}, {Enum, :reduce, 3, [file: 'lib/enu
m.ex', line: 3023]}, {Cluster.Supervisor, :get_configured_topologies, 1, [file: 'lib/supervisor.ex', line: 62]}, {Cluster.Supervisor, :init, 1, [file: 'lib/supervisor.ex', line: 
57]}, {:supervisor, :init, 1, [file: 'supervisor.erl', line: 295]}, {:gen_server, :init_it, 2, [file: 'gen_server.erl', line: 374]}, {:gen_server, :init_it, 6, [file: 'gen_server
.erl', line: 342]}]}}}                                                                                                                                                            
21:54:50.014 [error] CRASH REPORT Process <0.371.0> with 0 neighbours exited with reason: bad return value ok from 'Elixir.NoSlides.Application':start(normal, []) in application_m
aster:init/4 line 138                                                                                                                                                             
21:54:50.014 [info] Application noslides exited with reason: bad return value ok from 'Elixir.NoSlides.Application':start(normal, [])                                               
** (Mix) Could not start application noslides: NoSlides.Application.start(:normal, []) returned a bad value: :ok                                                                    
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed                                                          

[bitwalker]

Agreed, I don't know that it is practical to have super friendly messages for every possible missing config item, but we can definitely catch the major ugliness like in your example.