More user-friendly Slack integration messages

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Description

Right now, if we type /gitlab <valid-project-name> or /gitlab <project-name> <unknown command> it says,

GitLab error: project or alias not found

or

GitLab error: project or alias not found
Unknown command, these commands are available
...

respectively.

Proposal

Both /gitlab <valid-project-name> and /gitlab <project-name> <unknown command> should produce something like:

Unknown command, these commands are available
...

Thanks!