Raise if the user supplies a -C param and it doesn’t exist

[alxgsv]

Added config file existence check as discussed in comments of #3735.

At first I tried to move config file fallback logic from config_parse, because it's not really parsing of a config. But then I thought that it would be more straightforward to just check file existence when optparse catches -C: easier to understand the error when it happens, easier to preserve config file fallback, and easier to understand this logic when/if modifying it later.

[Tensho]

@alxgsv Hi, thanks for your effort 🙇 I guess you have to incorporate the guard check to this place:

%w[config/sidekiq.yml config/sidekiq.yml.erb].each do |filename|
  opts[:config_file] ||= filename if File.exist?(filename)
end

and it would be more appropriate to move the stuff to validate! method. It's a good practice to leave options parser with only typecasting (as a maximum). Please cover the check with a test here.

[alxgsv]

@Tensho thank you! I completely agree that it's a good practice to leave options parser with only typecasting. That's why I decided not to touch code you've mentioned. It's in the parse_options method but has nothing to do with option parsing. And it leaves no chance to observe original command line state at the validation stage (other then creating some additional flags, checks, etc).

So I decided to touch as less code as possible to fix the bug, but as you said it's not ideal solution too. It's better to close this pull request, because I don't think I can fix this bug in other way without refactoring of parse_options, but I have no time for that.

Please ping me if I should open issue for that!

[Tensho]

@alxgsv @mperham Fixed in #4054, does it make sense to you?

[alxgsv]

@Tensho for me every solution which throws error in this case works :-) Thank you very much!