[V3 Core] Set nickname not changing

[TheWyn]

Please be sure to read through other issues as well to make sure what you are suggesting/reporting has not already
been suggested/reported

Type:

• Suggestion
• Bug

Brief description of the problem

On 3.0.0b11, when using [p]set nickname Test, the bot's nickname doesn't change to Test. Running Red in debug mode doesn't show any error's.

When using [p]set by itself, it outputs the following error:

[palmtree5]

ya, both issues confirmed. Will fix for next release

[TheWyn]

Adding onto this bug if a [p]set sub-command is entered incorrectly a trace is given instead of the content of [p]help set.

Trigger this by doing [p]set nick test (nickname is correct command) and receive following as trace:

Exception in command 'set'
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/discord/ext/commands/core.py", line 62, in wrapped
    ret = yield from coro(*args, **kwargs)
  File "/usr/lib/python3.6/site-packages/redbot/core/core_commands.py", line 544, in _set
    admin_role.name, mod_role.name, locale
AttributeError: 'NoneType' object has no attribute 'name'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/discord/ext/commands/bot.py", line 886, in invoke
    yield from ctx.command.invoke(ctx)
  File "/usr/lib/python3.6/site-packages/discord/ext/commands/core.py", line 923, in invoke
    yield from injected(*ctx.args, **ctx.kwargs)
  File "/usr/lib/python3.6/site-packages/discord/ext/commands/core.py", line 71, in wrapped
    raise CommandInvokeError(e) from e
discord.ext.commands.errors.CommandInvokeError: Command raised an exception: AttributeError: 'NoneType' object has no attribute 'name'

[Kowlin]

Fixed in #1494