Misleading "notfound" error [JIRA: RCS-386]

[JMLX42]

This is a follow up on #1329

When the riak-cs admin user set in the conf file does not match the actual admin user credentials, riak-cs crashes with a very generic "notfound" error:

[root@aaf0e279323a /]# riak-cs console
config is OK
-config /var/lib/riak-cs/generated.configs/app.2017.01.18.09.38.29.config -args_file /var/lib/riak-cs/generated.configs/vm.2017.01.18.09.38.29.args -vm_args /var/lib/riak-cs/generated.configs/vm.2017.01.18.09.38.29.args
Exec:  /usr/lib64/riak-cs/erts-5.10.3/bin/erlexec -boot /usr/lib64/riak-cs/releases/2.1.0/riak-cs               -config /var/lib/riak-cs/generated.configs/app.2017.01.18.09.38.29.config -args_file /var/lib/riak-cs/generated.configs/vm.2017.01.18.09.38.29.args -vm_args /var/lib/riak-cs/generated.configs/vm.2017.01.18.09.38.29.args              -pa /usr/lib64/riak-cs/lib/basho-patches -- console
Root: /usr/lib64/riak-cs
Erlang R16B02_basho8 (erts-5.10.3) [source] [64-bit] [smp:4:4] [async-threads:64] [kernel-poll:true] [frame-pointer]


=INFO REPORT==== 18-Jan-2017::09:38:30 ===
    alarm_handler: {set,{system_memory_high_watermark,[]}}

=INFO REPORT==== 18-Jan-2017::09:38:30 ===
opening log file: "/var/log/riak-cs/access.log.2017_01_18_09"
Setup running ...
Directories verified. Res = {[ok],[]}
Setup finished processing hooks ...
Eshell V5.10.3  (abort with ^G)
(riak-cs@127.0.0.1)1> [os_mon] memory supervisor port (memsup): Erlang has closed

=INFO REPORT==== 18-Jan-2017::09:38:31 ===
    alarm_handler: {clear,system_memory_high_watermark}
[os_mon] cpu supervisor port (cpu_sup): Erlang has closed
{"Kernel pid terminated",application_controller,"{application_start_failure,riak_cs,{notfound,{riak_cs_app,start,[normal,[]]}}}"}

Crash dump was written to: /var/log/riak-cs/erl_crash.dump
Kernel pid terminated (application_controller) ({application_start_failure,riak_cs,{notfound,{riak_cs_app,start,[normal,[]]}}})

/var/log/riak-cs/access.log.2017_01_18_09 is empty.
Here is what /var/log/riak-cs/crash.log says:

2017-01-18 09:38:31 =CRASH REPORT====
  crasher:
    initial call: application_master:init/4
    pid: <0.148.0>
    registered_name: []
    exception exit: {{notfound,{riak_cs_app,start,[normal,[]]}},[{application_master,init,4,[{file,"application_master.erl"},{line,133}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,239}]}]}
    ancestors: [<0.147.0>]
    messages: [{'EXIT',<0.149.0>,normal}]
    links: [<0.147.0>,<0.7.0>]
    dictionary: []
    trap_exit: true
    status: running
    heap_size: 376
    stack_size: 27
    reductions: 119
  neighbours:

The expected behavior would be to have a more specific "admin_user_notfound" error.