Brigadier: Sometimes the error message is not useful

You input a command with an unknown name, or a missing but unrequired command, then the error message is not useful for it.
For example:

You input: /unknown
Error message: Unknown command at position 0: <--[HERE]
Expected: Unknown command "unknown". Type /help for help.

You input: /kill
Error message: Unknown command at position 4: /kill <--[HERE]
Expected: Missing variable: /kill <target> or "<target>" is a required variable.

Affected versions include earlier 1.13 snapshots, maybe since when the command system is changed.