Improve error message when CLI option doesn't exist

[jcamiel]

When using a non existing CLI option:

$ hurl --foo
error: error: unexpected argument '--foo' found

  tip: to pass '--foo' as a value, use '-- --foo'

Usage: hurl [OPTIONS] [FILES]...

For more information, try '--help'.

The error message is error: error: unexpected argument '--foo' found (error is duplicated twice)