Be more explicit about grade/confinement combinations and their impact

[Daniel Holbach]

I received this in an email from an upstream who got confused about the options available for publication:

------------
* On the http://myapps.developer.ubuntu.com/ website, after I uploaded
my very first snap manually, when I eventually pressed the "Publish"
button, I somehow got the following error message:

    Revision 1 (strict) cannot target a stable channel (stable, grade: devel)

  which was rather confusing because I did *not* target the stable channel.
  Later, I realized that targeting the "candidate" channel would
trigger the same error.
  Perhaps the "candidate" channel was added rather recently, and the
error message
  has not been updated to reflect that? I would rather it say
something like this:

        Revision 1 (strict) cannot target a stable or candidate
channel (stable, grade: devel)

  so newbies like me won't get confused.
------------

Stephen Stewart says:
<beowulf> "Revision #1 cannot target a stable channel as it's <a href="/help/grade">grade</a> is set to 'devel'"
<beowulf> i think
<beowulf> dholbach: if you can include a link to a help page for grade ^^ in the bug

[Daniel Holbach (dholbach)]

This is a bit blocked on https://bugs.launchpad.net/snapcraft/+bug/1629215

[Daniel Holbach (dholbach)]

Or https://bugs.launchpad.net/snapcraft/+bug/1630914 rather.

[Daniel Holbach (dholbach)]

Once the PR gets merged, http://snapcraft.io/docs/build-snaps/publish would be a good place to link to.

[Stephen Stewart (stephen-stewart)]

8:26 AM <beowulf> "Grade / Channel mismatch: a devel grade package cannot target a stable channel. Please check your yamls."

(another option, with no help link)

[Daniel Holbach (dholbach)]

https://github.com/CanonicalLtd/snappy-docs/pull/14 was just merged - http://snapcraft.io/docs/build-snaps/publish should be updated soon.