Allow commands to return a simple error

[vanstee]

Right now if you return an error from a command, the bot prints the following message:

An error has occurred.

At `2016-05-12T20:36:16Z`, @vanstee initiated the following pipeline, assigned the unique ID `ec536613be7840189c133a932f10eaeb`:

   ` help wut`

The pipeline failed planning the invocation:

   `operable:help wut`

The specific error was:

   Whoops! An error occurred. Command "wut" does not exist

That's great for when a command unexpectedly crashes, but if you mistyped something it would be nice for the bot to just say:

help: ERROR: Command "wut" does not exist

[vanstee]

We also need to allow for rendering error templates as well as raw text.

[kevsmith]

Design:

Add a new response status called "abort". Add support to Relay, pycog3, and cog-rb to allow commands to signal when they wish to abort by writing COGCMD_ACTION: abort\n to stdout. When Cog receives this status it will render the body (with template if the command has specified one) and then terminate the pipeline. A "pipeline aborted" audit message will be written as well.