Email UI shouldn't allow mixing of command and non-command text

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Furthermore:
 * There should also be a way to force the email interface to stop
   processing commands.
 * Illegal commands should be reported as errors rather than
   ignored.
 * There should be no ambiguity between the start of a message text
   and a message which starts with commands. To achieve this and
   still support naive users just sending text, it would be necessary
   for messages modifying a bug's status (which start with commands)
   to be sent to a different address or to have some special mark at
   the start.

I have a nice example recently of the problems caused by the ambiguity
between commands and text. When I filed
https://launchpad.net/bugs/29569 I got a message back saying:

] An error occurred while processing a mail you sent to Malone's email
] interface.
]
] Failing command:
] status and modification information features.)
]
] Error message:
]
] The 'status' argument expects 1 argument(s). It got 4.

Thanks,
Ian.
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